Well here are some of the things I've been made to do in a long distance relationship (some are the sort of things already mentioned in other posts above):

1. Have a schedule of fixed tasks like sleep with a dildo beside my pillow, and stick it in myself the moment I wake up (this can be really hard to do!), fall asleep everyday while fingering my hole, masturbate for 20 minutes every night before I go to sleep but never cum until permitted, etc.
2. Have a set of rules that must always be followed like never wear underwear (with exceptions - I have to wear women's g-string under my shorts for sports), always strip naked the moment I get home and remain naked there at all times (unless guests show up), always wear a buttplug to work and keep it there for half a day, etc.
3. Write daily reports about my horniness and my favourite/latest fantasies and email them over.
4. Take pics of myself in various outfits (some started out as normal clothes but altered to enhance 'sluttiness') or in lewd poses and email them over. The outfits and poses are usually explicitly prescribed.
5. Do 'dirty' things like smear my precum on a colleague's desk or computer (if chatting/camming from work - I'm sometimes told to stay back late for this).
6. When camming, sit like a slut, abuse my tits and hole, or bend over the desk yelling for Master to use me.
7. Wear skimpy shorts for a drive and while in the car, play with my ass through the leg-hole while moaning like a slut into the mobile for Master to hear.
8. Well, you get the idea...

There are loads of things that can be done, especially if you like to push your sub (if you're the Master) or be pushed (if you're not). The sheer thrill of doing 'dirty' things for your Master can easily negate the distance factor, at least for a while.