I had made the assumption that TRIM was enabled by default nowadays. Apparently not. To enable it for your filesystems, edit /etc/fstab and add "discard" as a mount option:
/dev/mapper/vg_foo-lv_home /home ext4 defaults,noatime,discard 1 2 /dev/mapper/vg_foo-lv_swap swap swap defaults,discard 0 0
Might also want to add noatime, it's not really needed.
To enable it for LVM, edit /etc/lvm/lvm.conf and change
issue_discards = 0
issue_discards = 1
To enable it for dm-crypt/luks, edit /etc/crypttab and add discard to the option column:
luks-33ed4f53-a649-b6b5-b871-7de44efdf1aa4 UUID=luks-33ed4f53-a649-b6b5-b871-7de44efdf1aa4 none luks,discard
A word of warning though, enabling TRIM with disk encryption may be a security risk.