1. Smile at them. Genuinely, not creepliy or forced.
2. Offer a genuine and unsolicited compliment.
3. Tell them they're important to you (in a way that won't sound weird based on the nature of the relationship).
4. Show them that they're important to you (same guidelines as above).
5. Thank them for something they do, or are. Make it meaningful.
6. Do something nice for them without expecting anything in return.
7. Write them a heart-felt note or a card. Not a status or a tweet. Good old pen and paper.
8. Praise/compliment them to someone else whose opinion you value. It feels good to hear someone compliment you to others, doesn't it?
9. Express genuine interest in learning about something that's important to them - a hobby, a job, a passion of theirs, etc.
10. Appreciate them for just being them. Express this to them however you can do so best.