Theres another thread with some info. Squash your buds first, the denser the starting material, the larger the yield. Remove all small leaves, and make sure that its all bud, no sugar leaves, they seem to absorb the oil back into the bud if you dont remove all the leaves. The hotter your press, the more stable your rosin will come out, lower temps provide more terps, but the rosin is unstable and hard to collect. I use my hair straigthner between 25-30 heat setting.
Indica's press much better, since sativas tend to not make very greasy hash i usually dont bother pressing sativas. The best yielders ive pressed are Kush varieties, Blue crosses, and anything with OG juices like a champ. If your buds are airy, and loose its almost not worth pressing unless there coated, if you do have larfy stuff, clean off all the leaves, and stem then squish the bud into a small pea shaped nugglet, this will allow you to get the most out of your non dense flowers. I also never press more than .3 usually around .2, any more and i feel the oil is reabsorbed into the flower. I use a 12'' G clamp that exerts a max force of 125 lbs, its perfect size and anymore force and i would break the straightner. 12 bucks at homedepot