Good practice for using a circular saw

For cutting porcelain or ceramic tile, standard practice is to use:

• A wet saw with a diamond blade
• A manual tile cutter (a carbide wheel slides along a rail to score the tile, then you snap on the line.
• Tile nippers for complex shapes

You can use a diamond blade with a hacksaw to cut holes, but it won't be very efficient.

If you have mostly straight lines, you can get away with a decent manual cutter and perhaps a pair of nippers. A wet saw makes the job go faster, but is not necessary.

Back in the day when I used to put up vinyl siding, I found that the best way to cut it with a circular saw was with a cheap 140 tooth plywood blade installed backwards (to give the teeth a negative rake and prevent chipping):

For really hard plastics, I've used a diamond abrasive blade before and that did a great job (these are incredibly expensive if we're talking about a single cut though). I would imagine an abrasive blade for cutting metal would do just as well though:

You should be able to pick up either of these for less than $5.

Wear good safety glasses and do a trial cut first with whatever option you go with - plastic will chip long before it will bind the blade in most cases, and these can be both sharp and high velocity.