By spending time with them.
Because of the distance, obviously you cant see them all the time")...so, use the Phone, plan special visits, exchange little gifts and cards from time to time, thats always nice too, and on the occasions when you do see them, make it special in little ways, its the little things that can make all the difference;)
Chat on a chatting site. Try to express yourself as much as you can by saying beautiful words and throwing kisses at him/her. When you speak on the phone be interested in what it is being said to you and give each other support as much as possible. Positive thinking and good attitude is a plus for it to work. Trust and communication is also a big factor. Plan a day that is covenient for you two to meet and make it the best day ever. Affection, who does not like it lol! Hug and give each other kisses. Whisper to his ear just how much you wish the day would never end and talk about how the sacrifice of being apart will have its reward. Talk about what you would like your life to be like together and just dream out loud. If possible video chat and hug your lap top as if you are hugging him lol! Throw kisses at him. Flirt and enjoy each other as if you were together.
Call them up and talk with them. tell them you love them. Send them letters randomly just because you're thinking about them. Maybe send little gifts. be there for them as best as you can and when you do see them make the most out of your time. :)
Surprise with gestures that let him/her know that you indeed did listened to them. remember what they told you and are thinking about them. It can be a book, a photograph, food and wines, even something simple that however has a lot of meaning to the two ot you.
Things that remind the other person he/she is not being forgotten.
Do not over text or call 24/7,that can get to be annoying, instead there are more thoughtful things that you can use to show your affection.
fly to them if you have the time...
by yelling very loudly
I agree with imathinkin :)
Making her understand what she really means to you despite her being far away. Call and text her regularly, pour out your heart to her. Make her feel as if you're with her.