I use glass often to make flat fiberglass parts. It helps to wax it - sounds weird - but it works. A couple of coats, buffed each.
If you have a piece that is curved - you can use plastic packing tape - leaves lines, but easily sanded out of your finished product. Another tape is silver refrigeration/H-Vac tape. Wax, wax (5 times is best) buffing between each. If you are going to make a complex part - like my fender flares, you'll want some PVA. here's a quick link to those products;
http://www.uscomposites.com/moldrelease.html
Also have a few wedge shaped shims to help separate from your mold surface.
Hope you have gloves already - Harbor Freight is OUT!
I kind of run through the whole process here:
http://www.vettemod.com/forum/downloads.php?do=file&id=94 It coves the results of using different composite core materials as well as some of the fabricating details.
Keep on "Itchin' for Fun!"
Cheers - Jim