There are many ways, some amazing and only possible in Long Distance Relationship.
Tell her you miss her - put your male logical mind aside and let the feelings run over you for a while. Forget everything and express yourself. Tell her how you envision watching the sunset with her, your future with her, and things like that.
Send Surprise Gifts - There are many online service providers you can use to send her flowers and other gifts. There is hardly any woman in the world who does not like flowers. Similarly, order things to be delivered to her at her address.
Surprise visit - don't tell her that you are visiting her. Ask your common friend to drive her to her favorite place and she will be happily shocked. If possible, come from behind and do that special scene of hiding his eyesight with your hands.
Prepare special gifts for her - don't just buy things and gift them. Make something special for her and gift her while you meet. (One time I made a booklet out of filter paper containing his sketches, his favorite quotes, poems, pictures of her, my vision of a future with her - I cut, stitched, wrote and colored). Such gifts will take your place when you are away for months.
Plan your next meeting - When you are away for a few months, talk to her about your next meeting - when you will next meet, what you will do and how you will spend and manage time.
Proper conversation - talk daily whilst both are unfastened and at ease. Send special text at least thrice a day (don't copy-paste text).
Morning Call - Talk every morning, a minute or two is enough. Say you love her and tell her how amazing she is and also wish her a very good day.
Talk more about the future - don't always talk about being alone but talk more about the future when you are together.
Support her emotionally - share her frustrations, stress and pain. Hear what she had to say. Don't argue and don't be logical. Let her feel that you understand.