A few solutions:
1. Cheap and Easy - the Accuwedge AR-15/M16 ACCU-WEDGE | Brownells
2. Cheap but not Easy - bed the upper to the lower as mentioned above
3. Easy, but not Cheap - Demon Tactical Quick Pin Demon Tactical Quick Pin
4. Not as Easy, not Cheap - JP Rifles AR Tensioning Pin JP Rifles...