The slow burn
If you really want to give your girl a great orgasm, don't rush her!
On average, women take much longer than men to become aroused and to reach orgasm during sex. Many women are much more responsive to acts of intimacy like kissing, caressing and stroking than penetrative sex, so it makes sense to spend lots of time on the former.
The longer the build-up to sex, the better it feels for a woman. If you spend plenty of time stimulating her erogenous zones, as well as parts of her body that you never dedicate any time to (such as massaging her arms and lower back), her body will be flooded from head to toe with arousing sensations, making her much more relaxed to have an orgasm.