We're not going to mince words here: The hardest part of any long distance relationship is going weeks (over even months!) at a time without sex. Physical contact is important to feeling connected to your partner. Also, yeah, orgasms are awesome.
Getting sexy over Skype is a given, but it takes extra effort to make things feel as enticing as the real deal. Try mutually lighting a few candles to set the mood. Maybe watch the same, um, adult video together. Send her something sexy to wear for your next video chat. Maybe even take turns being the virtually dominant partner and telling each other what to do. Get as kinky or creative as you both feel comfortable with.
Experiment with different mediums. Don't limit steamy V-chat sessions to before bed, talk dirty in G-chat mid-day. Ask her to send you a detailed email describing her X-rated plans for you the next time you're together, or send a text message telling her to meet you naked on Skype at a specific time.
We've argued in favor of sexting before, but in an LDR, a hot visual every so often is crucial to maintaining sexual interest. Encourage her to send you a reminder, something to the effect of: "In case you forgot what you're missing…this is how awesome I look naked." Keep in mind that every woman is different. Some may not enjoy sending this kind of message, but might get a thrill out of receiving one, or vice versa, so definitely feel her out before going full-blown Anthony Weiner on her.