- Quit smoking (if you do, and can)
- Start eating more vegimite if you like it, good source of additional folate and iron, as well as all those B group vitamins
- Get more calcium in your diet.
- Start cutting back on alcohol and caffiene (not really nescessary, but if you start to eat like you are pregnant now, you'll get into the habit, and it won''t be such a change when you do get preggers)

When you do get pregnant start using a safe moisturiser like Palmers cocoa butter for stretch marks, or something similar. If you keep your skin soft and supple, you'll have lesss chance of getting stretch marks. There are a couple of samples in the Bounty mum-to-be bag you get at your first OB/midwife appointment.

That's all I can think of, but I didn't do any of those things until I was 3 weeks or so. That's the advantage of forward planning I guess.