This is a big topic but I'll try to outline some basics:

Mainsail trim with spinnaker:
I use three leech tails placed on my main. The middle one goes straight back from the hoist height of the head of the jib (right about at the mast hound), the second one goes mid-point between that one and the top of the sail, and the third one goes the same distance between the top two below the middle one. When sailing downwind with the spinnaker, I trim the main so that the leech tails are streaming back but flicking to leeward about every 3-5 seconds or so. Use your traveler and mainsheet tension together to control the amount of twist in the mainsail so all of these are showing similar activity. If in doubt, sheet harder. If the wind is light (on the F18) I usually end up traveling out 10 inches from center and leaving the mainsheet just snug. If it's blowing 10 or better, the traveler is centered and sheeted pretty hard.

Jib Trim:
We trim the jib to try and keep the tales flying. The angle you sail will affect the jib dramatically. Figuring out the proper downwind angle to sail in light air is tricky and experience is really going to be necessary to figure this out. If it's windy, the wind will dictate how high you sail and your decision will be whether or not someone should trapeze. The jib provides only a small amount of drive with the spinnaker up so we don't spend much time with it - but we do try to manage the sheeting tension and twist so that all the tales are flying. If it is very light and the breeze will not support the weight of the jib, we furl it out of the way so it will not choke off the little breeze reaching the main.

Spinnaker trim:
Luff tension, for optimum downwind power, should be set so that when fully hoisted, you can grab a fist-full of luff and turn your hand 90 degrees before the luff gets tight and prevents your hand from turning further. If you are going to try and reach high with the spinnaker (often in distance racing or when sailing to an offset mark) you should ease the tack line several inches to let the spinnaker rotate more. No matter the angle of sail, your crew should trim the sail so that the luff is on the verge of collapsing...in fact, it should curl inward occasionally - once every 5 seconds or so. It takes a bit of skill and anticipation to keep the spinnaker on this edge without being surprised by a wind shift causing a catastrophic full collapse - this will undoubtedly happen the second the crew takes their eyes off the spinnaker to look around.