One time, my ex-girlfriend went away for a week, and she wrote me a letter every day and gave them all to me when she got back. It was really cute because it was full of all her silly, mundane thoughts about what she missed about me.
If you go the snail-mail route, it’s always fun to include lovey things like lipstick prints or a spritz of your perfume.
Definitely have regular Skype sessions. You can have major flirty/sexytimes via Skype!
You can also send them cute surprise gifts via Etsy (just tell the seller the recipient’s address instead of yours). There’s a lot of sweet/romantic stuff on there, but you can also just send them useful things like nice soaps or mittens or whatever.
I dunno, I’ve never been in a long-distance relationship so I’m not sure what to tell you!