The best lubricant I've found so far is petroleum jelly.
Someone suggested it a while back on the forum, and I've been using it for a while, but only casting with epoxy resin. Now Jimshoe has reported that it works great, and he uses polyester resin.
Some people report good results with grease, but petroleum jelly is cheaper and pretty non-toxic.
The epoxy I generally use is slow-curing, and really likes to stick to molds. I've made some almost-cylindrical units using plastic food containers similar to the Rubbermaid type, and expected to have to cut off the molds. But no.
The slow epoxy needs to cure a few days, at least, before removing, but if I pull at the edges of the mold (any plastic mold) so as to break it away from the unit, then turn it upside-down and press down on the center, the unit will slide right out.
The jelly will leave a greasy residue on the surface that can be wiped away with some citrus-based or other solvent. Also, any smear marks left on the inside of the mold will show up on the surface of your unit. If you want the surface to look good, gently wipe away any excess jelly from inside the mold before loading. Only thing is, I don't think a thin coat is any better than canola oil; the advantage of petroleum jelly is that you can put on a thick coat.
I find that one thick coat of the jelly on my ice cube trays will last for several pours, saving myself time and hassle.
Unfortunately, my slow epoxy still loves to stick to "non-stick" muffin trays. The jelly does stick to the non-stick, unlike some lubes, but if I use a thin coat of jelly, and the slow epoxy, I still have to allow several weeks for curing, and then smack the mold upside-down to get a release. Smacking these trays bends them, so I cut out individual molds for making fine muffin units.
Update 1/21/4: Lately I've switched to using a faster, thicker epoxy, and no longer have the kind of mold-sticking problems I did. Now I'm favoring canola oil more as a lubricant; it does not leave as nasty a film, it does not leave smear marks in the resin, and it works fine for most things.
Update 7/2/4: I haven't used petroleum jelly in a while, but that's probably what I would resort to if I were still casting into metal pyramids or metal tetrahedrons. I've used coconut oil (happen to have some that's rancid) a few times lately, and it is quite good. And lecithin-based pan sprays work well, too.
Update 3/19/10: A reader enlightened me about car wax. He gets good release even from glass molds using epoxy. I tried it on a metal pyramid mold with epoxy, and it did work well.
Rub on, let dry, polish off.
These days I generally use a lecithin pan spray for flexible plastic or silicone molds.
Update 12/6/11: Coconut oil may leave a gummy residue which might contain compounds harmful to at least some types of rubber molds.