When it comes to birthdays, you shouldn't be shy about asking for what you want. Whatever you do, if you are asked for suggestions don't reply with "You don't have to get me anything." This kind of response is just going to add to the pressure on your partner who wants to buy you something special.
As for buying a present when you don't have an idea, here are a few suggested guidelines to follow:
Unless your girlfriend is a huge fan of rugby, don't buy her a ticket to see the Parramatta Eels take on the Melbourne Storm.
Don't choose a birthday to buy your partner the gym membership they have been saying they would get forever. Stay clear of diet cookbooks too.
If a present is worthwhile buying, then chances are it could be sold out. Don't run the risk and buy early.
A gift doesn't need to be expensive, but it does need to show that some thought has been put in. Don't just buy the first thing you see if it doesn't mean anything.
Try to look for something that only you would buy as the partner. Anyone can buy candles, chocolates or a tie, but only you can buy something that's intimate and pulls at the heart.