I second Evan's suggestion for a CCF pad. However, I would personally recommend one of the Gossamer Gear Nightlight sleeping pads (http://www.gossamergear.com/cgi-bin/gossamergear/xdpy/s/Sleeping/index.html). They are made from Evazote which I believe is the most thermally efficient type of foam. Not quite as cheap as one from Walmart but still reasonably priced. Another thing to note is that this pad is very comfortable for me when it is new but it slowly gets compressed over time. If you are buying this mainly for Philmont, then I wouldn't worry about it. Also, you are likely not as heavy as a full-grown adult so compression shouldn't be as much of an issue.
When I went to Philmont, I carried one pair of shorts, 1 pair of zip-off pants, and 1 pair of Capilene long underwear (for sleeping). I believe that even this was excessive and if I went to Philmont again I would take the 1 pair of zip-offs and the 1 pair of long underwear (Philmont requires a dedicated pair of sleeping clothing). I chose zip-offs for two reasons. First, I just really like wearing shorts and I find them more comfortable than pants. Second, at Philmont in the early morning it can be very chilly so pants are welcome, while later in the day it can get fairly hot and shorts make it more comfortable. Some people complain that the zipper on zip-offs can cause some sort of chafing or irritation but I have never had this problem. I have had good luck with the REI Sahara Convertible Pants.
The pack really depends on how much weight you will be packing, so many suggest that you buy it last. I would recommend checking out Gossamer Gear and ULA-Equipment for reasonably priced lightweight packs.
If you have not read this BPL article it is a MUST: