One option to consider to add flavour to a chicken is brining. I've personally never brined a chicken, but everytime I come across mention of it in a foodie blog, forum or elsewhere, it always seems to be considered a good way to impart additional flavour into the meat.
Since writing this answer I've followed Nigella Lawson's recipe for Spiced and Superjuicy Roast Turkey which has resulted in fantastically tasty and moist turkey at Christmas every year, to the point where I won't countenance turkey at Christmas without brining it now. If this translates as well to chicken (which I fully admit I should've tried by now!) then it's well worth the time and effort.
The only method I have personally found to be reliable for grilling/pan-frying chicken breasts to a relatively uniform doneness is to pound them very, very thin with a mallet or rolling pin. Thin, as in scaloppine-thin, so that it cooks almost instantly in the pan.
Every other stovetop-only method is almost certainly going to produce a bland, tough cut, regardless of whether you press it down or not. As noted in my comment, my usual (lazier) method that does not involve pounding is to get a nice sear in the pan, then jam in a temperature probe and bake it in the oven until it's done (the USDA recommends 165° F, I usually don't go quite that high).
If pressing the meat actually accomplishes anything at all, it would most likely be to just squeeze out whatever tiny amount of precious juices the breast does have, and possibly give you slightly more even cooking on the exterior only; it will not help to cook the interior much faster unless, as stated above, the cut has been pounded extremely thin and flat, at which point it doesn't really matter.
P.S. Salt and olive oil is a terrible "marinade" for any cut of meat, especially a chicken breast. The salt is just going to get suspended in the oil and never reach the meat at all, and the oil itself won't have much of an effect on such a lean cut. You really need to change your marinade as well, preferably to something water-based (or at least not 100% oil).
Best Answer
A thermometer is the only way to be sure.
For methods with a consistent level of heat (stove, oven), you should able to learn the average cooking time, and outwards cues of color and texture that match the right internal temperature. Remember that these will vary with cooking method and temperature. Internally, the meat should look opaque and white.
These are CUES of doneness, not guarantees. Use the thermometer.
For methods with inconsistent levels of heat (bbq, campfire), wait until it looks nearly done and then check with the thermometer.
Err on the safe side. Prepared properly, chicken is a lot more forgiving of overcooking than steak.