Alumni Brother from Eta chapter here (initiated Spring '03, graduated Spring '07)...
I'm also an alumni adviser (well, Assistant Alumni Adviser anyway), but for my social Fraternity, Kappa Sigma. To tell the truth, I didn't know PSP had alum advisers... I'll have to see what I can do, maybe get a little more involved there too. <- that might be a good way to get them involved - let them know that positions like this exist (unless you mean you're the Alumni Chair, and are still an active undergraduate ... in which case I'm totally mis-reading your position)
As far as getting alums move involved... Remember that a lot of us have 'real' jobs, families (or are trying to get one), are no-longer in walking distance from campus, etc. So just taking off some random weekend may not always be as possible as it was when we were undergrads (or even grad students). Like wise, while I won't turn down a good party, getting totally drunk isn't always on the top of the list of things we want to do (again, going back to real jobs & families and such). So make sure you schedule more than just parties...
Keeping in contact is useful. Send out emails & newsletters on a regular basis (monthly? at least semesterly), letting them know whats going on with your chapter - events, how many pledges you've got this semester, etc. Make sure to invite them back to big events, and to let them know they're always welcome to stop back whenever - even just to sit in on a meeting, or to hang out at a Big-Little event or show up and hang out with you when you do a trip to the Zoo or whatever you're doing... Homecoming, Initiation and Formal of course are big draw backs for alums, as is a Founders Day celebration - both national & your chapter's. Maybe get something going as a special event for your chapter that you can invite Alums back. At Eta, we do Dogs 'n Hogs, and have a pig roast then followed up with a hot dog roast at a local farm with hay rides, etc. While we do it more as an interchapter event, it could serve as a great draw back for Alums too. Ask them, see what they'd be interested in coming back to do too. Maybe if one of your alums is a lawyer, you could ask them to come back & do a scholarship event around the law ("What happens if I get arrested...") - its always helpful to know your rights in case something should happen & you find yourself on the wrong side of a court room... If they're local, that might even help get them some extra business (hopefully not a lawyer, but for some other professions - like web design or whatever).
Once you get them involved, keep them involved. Don't shrug us off like we're the special china you only bring out once or twice a year. Alums are around for more than just homecoming & formal...
Maybe set up a regular meeting of some kind. Once a month, my father's HS graduating class, at least those still local, get together for breakfast somewhere to talk about stuff. I don't know about Upsilon, but I know a lot of Eta has a large grouping of Alums around the Pittsburgh area so getting them together wouldn't be too hard. It'd help remind them that they are still Phi Sigma Pi Brothers, and all the good times they had as undergrads. Maybe send an active or two to the meetings to keep the Alum informed about stuff a little less formally than a newsletter...
We also started a thing where we would pair pledges with alum, and encourage each to exchange emails. This way the pledges could talk to someone who isn't around 24/7 in case they had an issue with something that was going on, and gain perspective from someone who's "been there, done that, got the shirt"... And the alums could remain a little active too.
Alright, that's probably enough... Good luck, & let me know if I can be of any more assistance to you