Originally Posted by TP
Try to be prepared in advance and suggest an alternate routing if you don't like what they offer.
This might be the most valuable advice (since that's what was asked). Look on some of the orbitz/etc websites before you go and find the routings that are closest to what you originally planned. Jot them down and "request" them as the agent starts looking. If they're on US, you're basically gonna get them. If they're on LH, AF, etc they'll consider those second (obviously still helps your US status - AND you might be able to double dip with US and LH mileage deposits into your US account).
Obviously you're stuck in a way by having to go to your original departure airport (PHL) and that limits your options but there is lots of service from there.
The other good advice is to use the load-factor website above, and also something like
http://flyaow.com/classavailability.htm to keep an eye on Envoy loads. The holy grail is the combination $600 voucher (cash) / upgrade and this other tool will help you find flights that have upgrade availability still. Note though that this will change alot in the last few days before the flight AND the list will start getting cleared the day of the flight so even a flight showing 20 seats in C the night before might be down to 2 or 3 by the time you check in (but you should still aim for that flight rather than one showing 5 or 6 open in C the night before because there are probably roughly the same number of FFs on board jockeying for upgrades in the summer and the one with 20 will likely still have some left - 5 or 6 probably not).
Hope all this helps - you got great advice. Good luck!