Definitely use your fingers first. This can help her muscles relax and open up a bit.
Try letting her be on top, and assure her OFTEN to go as slow as she wants (will likely require patience on your part).
Read the threads on preventing pain during anal sex. The same principles apply.
Try not to go more than 3 days without trying or she might get un-used to it (but always give her a day off if she needs it)
Depending on the size of your dick, and of her vagina, it will take more or less time, but unless (like wax_off said) there is something medically wrong, things will get fun soon
