Peter,
Here's one thread over at XDA that discusses apps2sd:
how to get a2sd working? - xda-developers (and specifically this post:
xda-developers - View Single Post - how to get a2sd working? is helpful).
Looks like a2sd / apps2sd (same thing) involves the apps2sd.apk app, partitioning your /sdcard to have an EXT2 or EXT3 partition, and possibly some busybox commands.
apps2sd is usually built-in / baked-in to a custom ROM and I've seen a few early custom ROMs where the devs didn't quite get things setup correctly and a2sd doesn't work. It does seem a little tricksy to use even when the ROM does fully support it (based on things I've read). There's also the issue of a2sd shortening the life of your /sdcard (due to the partitioning?).
I haven't seen an obvious or simple set of instructions for installing this on a non-custom, rooted phone like you are asking about. Perhaps the best bet would be to post a query directly on the XDA forums (
Droid Eris Android Development - xda-developers) and see if anyone can direct you there (they would know best since that's where the devs hang-out).
Good luck.