I fished the San Juan in August. My suggestion is to hire a guide for a half day and learn the rigging and presentation methods. Be prepared to test your mending skills to the limit. Size 24-26 flys are the norm. Small splitshot sizes 8-9's. Use 9' tapered leader with 6x flouro tied on to that about 2'. Place your shot right above the triple surgeon knot so it doesn't slide down. Use as light of strike indicator as possible as the strikes will be very subtle. Tie on another fly through the eye of the first fly with another 12" or so of 6x flouro.

While fishing during wading, watch your waders around the water line and see what the bugs look like and match the hatch. Pay attention to the fish to see what depth they are active at and fish the appropriate depth.

While I was there I never went below the T-Hole. I loved fishing the braids and the upper flats in the small holes and pockets. Sneak around the braids and target fish that are staged up at feeding positions. You usually only get one chance to hook them if they bite so make it count. Get a decent sized landing net too. The fish can be 18-20" pretty common.

Abe's seems to have a pretty good handle on what's happening on the river. They have a pretty good sized selection of flys too. Don't forget to try your luck fishing small dry flys in the flat water and back sloughs. Look for cruising fish sipping bugs and cast in front of their path and they will bite something close to what they are feeding on.

Hope this helps you out a little,

Rich