For the 25,000 miles in bonuses I got, it was worth using EasySabre to find the best flights and then booking them through United Connection. Also, you can put UC into "expert mode" and it will show you the number of seats left in each fare code.

Once you book it, when it gets close to the day of the flight UA opens up remaining seats to all fare codes and you can call the PremEx hotline and get your flight switched for no charge.

WARNING: you cannot change the routing or change to or from codeshare flights!