Be courteous and considerate, have a good attitude, and communicate with the other folks there.
Offer a hand with the big stuff if you see folks need it and ask for a hand you need one. If its crowded, the good thing is that there'll be folks standing around who'll be glad to help carry your boat down to the water. Remember that beer is the universal currency down by the river.
Think about where you'll be staging your gear, airing up, rigging, etc. and make sure that you're not impeding the flow of traffic to/from the water.
Whenever I'm going to a busy launch area, I try to prepare/arrange things ahead of time so there's less rigging at the put in.
Once rigged, move your boat off the ramp and tie it up someplace out of the way.
Don't get swept up in the frenzied go-go-go mode that you'll run into at Shoshone or other really busy places - that's not what we boat for.
Throw down for an old snowmobile trailer and arrive at the put-in rigged and ready to go.
Other things - If you're running Shoshone or Grizzley Creek, you can call the rangers ahead of time and they'll know when big groups of commercials are launching so you'll know what time to avoid. Eagle Ranger Dist # (Shosho) 970-328-6388 (# may be old). You can probably do the same at Brown's.
Go boat less popular areas. If you're a novice rafter, then the float below Glenwood Springs to Newcastle will keep you busy on the oars but not be too challanging (just run the South Canyon Wave(train) right down the middle). Launch from Radium and run to State Bridge. Run from Cottonwood Island to Dotsero
Remember the three rules of rafting and have fun.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse