Buy some bel-ray waterproof grease (or whatever brand grease, just make sure it's waterproof). Get some rubber gloves and work the grease into the bearings as best you can. Don't let any dirt or grime get in there. Then back the entire rear carrier with as much grease as you can get in there. Don't make it so much that it's under lots of pressure because that may blow the seals out, but get in lots so that the grease can get to the bearings. When you're riding, the grease gets hot and can flow a little bit....make sure there is plenty!! Oh, and obviously clean everything up really well, you don't want any contamination in the carrier there. Also, with your greasy hands, or gloves, put a little bit of grease on the inside of the seal, the part that touches the axles. This will help it seal well, keeping the grease in and water/mud/dirt/sand out and make it so that you won't have to do this again for a long long time. One more good trick is to put a dab of RTV on the outside surface of the seal (not the part that rides on the axle, the part will be touching the carrier). This isn't required and most dont' do it, but it will make those bearings last longer and keep bad junk out/grease in.
Lastly, if you want to make it a grease-able setup, get some grease zerks, get a drill and tap of the proper size and you can tap the top of the carrier while you have everything apart and then thread that zerk in there. DO NOT do this with everything together because the metal shavings from drilling the hole getting into the bearings will totally defeat the purpose of the grease zerk in the first place!! That way you will be able to fill the carrier up with grease every so often. If you do this, don't grease it every ride because every time you use a grease zerk you introduce a little bit of grime. Just grease it once every few months or whatever, depending on your riding schedule.
