look in /etc/fstab and see if there is line like:
/dev/hda3 swap swap *options* 0 0
If not add one, replacing hda3 with the partition your swap is, and the options with anything you like, mine are "pri=42" but I don't know what that means. Look at a partitioning site for information about options.
Gentoo handbook recommends replacing the first "swap" with "none" and "*options*" with "sw". ie:
/dev/hda3 none swap sw 0 0