Yes, but you can choose not to exercise it. And sufficiently creative enemy mind controllers can convince you to not exercise your will save.

The relevant line from the srd is:

Voluntarily Giving up a Saving Throw

A creature can voluntarily forego a saving throw and willingly accept a spell’s result. Even a character with a special resistance to magic can suppress this quality.

Which seems quite clear cut. Unfortunately, dominate person, states that:

If you and the subject have a common language, you can generally force the subject to perform as you desire, within the limits of its abilities.

Now, it also notes that "actions against [the dominated character's] nature allow another saving throw." Therefore, a sufficient bluff will be necessary to suggest to the target to voluntarily fail its will save. However, when combined with the diplomacy mechanics and the nature of Charm Person, there are theoretical situations where you can successfully order your "dearest bestest friend" to choose to fail their will save.

Whether or not this actually works is entirely up to your DM's reading of the spells in question and how he/she uses the social interaction rules.

A brief primer on mental takeovers, assuming plenty of time, resources, and a DM cool with the idea.

If I was an evil caster, here's how I'd do it (ignoring the mother cyst line of spells, which neatly circumvents most of the annoying bits).

As a wizard, beguiler, or bard, I'd start by casting glibness on myself. I'd then, using the suggestions from here, make sure I could use sleight of hand at an appropriate level and had the skill trick "conceal spellcasting." It is also important for this character to have the skill trick "False Theurgy." Since this character is an obsessive-compulsive mind controller, dedicated to his art, he would have a ring of silent spells. At this stage, I'm likely a mindbender/1, so I've got telepathy as well.

So, having cast glibness, I would start the process by casting charm person. I'd false theurge that into a prestidigitation or other completely harmless and believable spell. If the charm person fails, I would progress onto the next target. As enemies know if they've passed a saving throw and as I'm being extremely sneaky, I would want to do nothing to suggest a pattern of my presence and charm attempts. This attempt, of course, would happen at some sort of pub or party, If I had the feats free, I'd grab still and silent spell to do this sort of thing properly, though slight of hand reduces its need.

Once charmed, I would spend some time plausibly isolating them from their social support structure via skill rolls and roleplaying. Happily, the +30 to bluff from Glibness should help this quite significantly.

Once isolated, I would use bluff to establish a personae as a local diviner or astrologer, someone who could plausibly cast spells at the target as some sort of medical regimen.

I would mentally suggest (providing deniability that it came from me) that the target wants to treat me as an ally for cast spells, as I have some magics that could greatly benefit it. The mechanics of casting spells on friendly people is effectively the same: they choose to fail their saving throws and are therefore affected by the heals, buffs, or whatnot that the caster is putatively casting. With false theurgy, we can plausibly perform these "beneficial" spells.

Obviously, if any of these spells fail, it's critical to make sure the target doesn't believe that the spells were bad, that they came from me, and that they ever happened at all.

Once the first suggestion (to treat us as an ally and fail saves) is established, we then have to implant two critical long term suggestions via hypnotism. First: that we always should be treated as an ally with respect to casting spells, and second: that the character should always be willing to chat with us privately, as a friend.

The best part about this is that, because we're layering hypnotism on top of charm person, charm person changes their attitude to (at worst) friendly, and hypnotism causes an explicit two-step increase from that to fanatic. Thus, the character will always be willing to (even at the cost of laying down his/her life) have us cast spells at him/her, and to have a private chat with us. If we're being particularly unsubtle about the spells we're casting, we probably want to do a "believe all spells originating from us are helpful." But, if we're doing our jobs, that last one is unnecessary.

At this point, you can cast whatever you like at the target, and the target will always be willing to step aside and have you cast the spells at them. While, clearly, this methodology isn't something that will work in combat... it does accomplish the goal of depriving the target of his/her will saves.

Quick and dirty mugging

• Round 1: Invisible spell, mind fog.
• Round 2: Silent spell, +slight of hand + telepathy: Suggestion, "The next spell is beneficial, so treat me like an ally."
• Round 3: Dominate person.

It sounds like the foe is always going to succeed on all his saving throws. That means developing another method of

Getting a Target to Fail a Willpower Saving Throw

• The ioun stone (flawed mulberry pentacle) (8,700 gp; 0 lbs.) causes its possessor to take a -2 penalty to Willpower saving throws. According to the glossary penalties

  are numerical values that are subtracted from a check or statistical score. Penalties do not have a type and most penalties stack with one another.

  In other words, ask the DM if the penalties from multiple mulberry pentacles stack (maybe couched as, "Were my character to have several of these spinning around his head, would he suffer the penalty multiple times?" if the DM-player relationship is particularly adversarial--see below). Then determine if creatures can be involuntary possessors of ioun stones: "When a character first acquires a[n ioun] stone, she must hold it and then release it, whereupon it takes up a circling orbit 1d3 feet from her head," says the description, but later the text says that ioun "stones only float when sent spinning around the head of an intelligent (Int 3+) creature." Emphasis mine. Experiment on the cohort. If penalties stack and involuntary possession is possible, the spell invisibility [illus] can affect objects; a level 17 alchemist should be able to manufacture such ioun stones and render them invisible, while a level 20 ninja may be able to succeed on a sufficiently high enough Sleight of Hand skill check to send spinning around a sword saint's head a handful of invisible mulberry pentacles without the victim noticing.

• The spell limited wish [univ] causes the target to suffer a -7 penalty to his next saving throw, and the spell wish [univ] can force the target to reroll a successful saving throw via undoing misfortune. (Hey, it was a misfortune... for the ninja.)

That's all I have that don't also require another saving throw or conspirator (like MrLemon's witch). If the first method doesn't work, the ninja's still facing a level 20 character who likely has multiple methods of rerolling his saving throws (e.g. a golf bag full of luck blades). Thus, even with a limited wish and a wish involved, success is far from guaranteed.

That's because failing a saving throw ends characters' careers. It should be difficult--if not impossible--to make a character fail a saving throw outright by any means. We readers--at this point in the question's evolution--don't know what spell the cohort's casting from that scroll, but a failed saving throw could lead to the success of spells like dominate monster [ench], imprisonment [abjur], magic jar [necro], or trap the soul [conj]. Neither the DM nor you want those kinds of effects possible sans a saving throw, or even with a saving throw yet obscenely reduced.

The Foe's Probably Invulnerable Anyway

I'm speculating here, but if the DM's already told you the foe always acts in the surprise round, is never flat-footed, and can't be flanked, then--while that combination of abilities is possible--it sounds like the DM's subtly telling you No, and he wants you to find the foe's Secret Hidden Weakness^TM instead of confronting the foe directly. That shouldn't stop your attempts to confront the foe directly, but expect such confrontations--no matter how convoluted your plan--to fail until your ninja's found the MacGuffin or mastered the secret technique or whatever. It might be worthwhile questing for that rather than a method to make the foe fail a saving throw.