density altitude is "pressure altitude corrected for nonstandard temperatures" your thinking pressure altitude which just figures in the barometric pressure. to get density altitude you first need pressure altitude, thats pretty easy to get.
standard pressure is 29.92, every inch is worth 1,000 feet. so if you have a barometric pressure of 30.05 pressure altitude would be 113 feet lower than actual elevation. so the affects of pressure altitude are pretty minimal.
finding density altitude is not so simple. standard temperature is 15 deg. C at sea level, thats not average temp, thats standard temp, i don't know who came up with that stuff... there is a normal lapse rate of 4.5 deg. for every 1,000 feet of elevation. so basically standard temp. should go down 4.5 deg. for ever 1.000 feet higher you get.
SO if your at 2,000 feet standard temperature would be 6 deg. C anything hotter than that will give you higher density altitude, and worse air.
i don't remember the exact calculation for finding exact density altitude, i have an aviation calculator (e6b) that does all the hard work for me. so if anyone is really intrested in this stuff go to a small airport and pick up one of them, they cost like $6. OR do a search and i'm sure you can find the equasion. or you can also go to summit and pay a few hundred bucks for a weather box that does it all for you.
humidity also has its effects, but i don't know of any ways to findout how it effects anything. i know high humidity is usually a bad thing, but if your making boost it might help with some detionation. i don't know about that.