From the SD card to the phone's internal memory? Quite frankly, you can't. But fortunately so. The Eris isn't like newer phones with gobs and gobs of onboard memory. You get 150 MB maximum usable memory in some of the more optimized ROMs, and that doesn't include apps.
EDIT: Oh dangit I thought I was in the Eris forum! I guess that's what I get for having multiple tabs open!
But I'll leave this here because this applies for some other older Android phones as well. Like the Hero, which is essentially the same as an Eris. Some other lower end phones have this limitation.
Still, I don't know why you would NOT want them on the SD card, unless your SD card is very small.