Oh, and quit doing slackers' work for them. They'll never develop a work ethic that way!
(I can say that because I'm slacker extraordinaire.
I just get away with it because I'm very good at the Hail Mary heroic effort - I don't ditch work onto anyone else!)
Edit to Add: Rob mentioned that you're a nice guy - which is completely true - but don't let yourself be taken advantage of. The corollary is also important. If you're doing/supplementing someone else's work, likely your manager(s) don't know this, so you're not getting credit you deserve.