Someone once told me that to be an extraordinarily good Dom you need to spend some time as a Sub. I guess it wouldn't hurt (oh but it does... ha ha) to get into the head of a Sub and understand exactly what makes a Sub tick. I am not suggesting that you should switch and spend some time in submissive servitude to your own Dom but there's no harm in approaching some BDSM connoisseurs on RedHotPie for some tips.
To the uninitiated, BDSM sounds like it's all about whips, leather and ropes but the genre is highly multi-faceted. It can involve some hairy things which we can't actually even discuss at length on here. Some of the less hairy things Doms and Subs enjoy include the use of TENS machines, medical equipment and racks/harnesses to provide extra uncomfortable restraint. A lot of these things can be googled at your peril/delight! See what you think you might like to try.
For some practical advice, do make sure you and your Sub have discussed the upper limit of her interest in having pain inflicted on her. Some Doms may suggest that this is extending too much courtesy to a Sub but since you're new at this, I think it's best. Your Sub might be able to take the pain but are you going to be 100% comfortable? Being new, I'd consider what your upper limit is as well.
Also, do agree on a safe word which can be used to slow the action down if something isn't going right for your Sub. If you use restraints and gags and your sub can't say the safe word, agree on using a nod or head shake as a safe signal. Take things easy and learn as you go. I'm fairly sure no real Dom worth his/her salt has learnt the art overnight.