You could check www.farecompare.com for a fairly exact idea.
 
EDIT: After a quick peruse, it looks like you’re looking at btwn 800-1000ish. Sometimes airlines do run deals though, so it pays to keep an ear open.

I’m not really seeing anything good after late May, but checking regularly and pulling the trigger when you find a good fare are key.  Last week some great fares from SFO to Frankfurt showed up and then quickly disappeared – if you find a good deal jump on it immediately.  Check back here to see if people post fares they find; I check Bay Area fares quite often and will post anything I see.  How long are you planing on staying in Europe?

For about 3-4 weeks. Any idea what cities are generally the cheapest to fly into? London seems to be the cheapest…

Generally speaking London is among the cheapest if not the cheapest, and good fares from SFO to Frankfurt and Munich are pretty common as well.  However, you never know when good fares to other cities might pop up.  

Jason Travels does a charter to Zurich that might interest you, www.jasontravel.com/… Otherwise, have you tried the links and strategies in our new user thread (top of this forum)?

For example… I guessed your stay at ~ 2 weeks. ITA shows Munich $1366, Rome $1389, Hamburg $1339, Frankfurt $1224, Shannon $1155, London $1113. Using the cheapest of these, running it through Bookingbuddy on specific dates turned up $976r/t total.

Farecompare might also be helpful.