Before putting the gasket on, put the valley pan on without gasket and check to see if there are any gaps between the lip of the valley pan and the heads and front and rear of the block. It is VERY important to fit with out gaps. After it is flat with no gaps I use Permatex High Tach Sealant to glue the gasket to the pan. Then I put a thin layer of RTV sealant on the heads and front and back of the block where the gasket will sit. Torque the bolts to 15 ft lbs.
Making sure the shape of the cover fits the contours of the block and heads is very important to get a good seal. If you've got a round nose pick body hammer, it's a little easier but a plain old ball peen hammer will work too. Just set the cover on the block without a gasket, start the bolts enough to keep it from moving around on you, and work your way around the edge of it, fitting it to the block/heads by tapping the edges with the hammer as you go.
Another tip: before you put the gasket and pan on, dab some silicone sealer in the 4 "corners" of the block where the heads and block meet, enough to fill those little gaps. Let it set up for at least 30 minutes before you continue. Sometimes you can get leaks in those little gaps, especially if for some reason you're having to run thicker than stock head gaskets.
Before you insert the bolts, dab some silicone sealer underneath the bolt heads to help seal the bolt holes in the cover.
Tighten the cover down slooooowly. Sometimes tightening it will tend to push the gasket out, especially on the long sides on the heads. If that starts to happen, just stop and use a wide, flat tool (like a big bladed screwdriver) to ease the gasket back under the cover. Then continue.
Once you've got it tight and you're happy with it, it doesn't hurt to dab some more sealer around the circumference of the bolt heads to seal them to the cover.