Hmmm you just spend time with them, that's how I keep mine anyway. Of course, they should be worth your time. That way you only keep good friends and easily detect the ones who only talk to you when they want something.
Funny enough I spend more time talking to my RL friends online than offline though!
My first attempt at a "Best friend" was like 21 years ago when I started school and stayed that way for a while but I also made another a few years later... about 17 years ago... and I relabelled them as "Joint best friends".
Remained close friends with them both through most of school, we were pretty much inseparable. Then things changed a bit with 6th form, the first studied different things and hung around different people and we kind of drifted apart somehow. It's weird looking back, one minute we were inseparable and the next we finished school and basically never saw each other again except once or twice a year by coincidence when the best we could manage was "Hi, how are you doing" as we no longer had anything in common.
Since then by coincidence I ended up working for the same company for a year, then he left. But the relationship had completely changed from 'friends' to 'colleagues who get on'. We've not been in touch since he left - started out best buddies but ended up leading completely different lives in the end.
Somewhere around this time the second one varied between "losing touch a bit", "fallen out" and "best friends", and I also made a third. No.3 and I had the same intellectual interests and went to all the same classes and stuck with each other all the time like the best of buddies.
Then the end of 6th form came, when No.3 moved to another part of the country and with the exception of one town event when he returned with fiance in tow and managed to say hi, he never spoke to me (or anyone else from up here) ever again and completely ignored all attempts to get in touch.
Throughout all this the one consistency (even though we had ups and downs at one point) was no.2, making it pretty clear that he was my true best mate. 17 years or so and still going strong :D Truth be told we don't hang out together much these days... he's always away attending events at our near-ish city, and whilst we used to hang around at his house on a Saturday night, I tend to be thirsting for ale a bit earlier and end up going straight into town - though we do meet up there.
I really should make more of an effort really, or we'll end up drifting apart (see, that's how it happens!). It's not like I'm uninvited to the city do's (though he's given up asking), it's just that I usually find some 'difficulty' or other that makes me say no... I have some pretty strong Avoidant Personality traits. The house thing is just me being selfish and a rubbish one-way friend - ought to make an effort.
As for online, I've found that online friendships can be perfectly real but they come and go. There are only so many times you can say "Hi, how are you today / how was your day" before the questions and the responses get old, and without actually meeting and doing stuff together there's not much to talk about and not much reason to talk to them other than to try and keep in touch - often pointless, as you're just prolonging the repeats of those awkward "Hi. How are you. Cool." moments.
Sometimes you get a good online friendship as you do things together in online worlds like Warcraft or SL or whatever, but then you move on to some other world.
I've also found that with my RL friends online (there's a bunch of us who have somehow ended up like a 'gang' and run a messageboard together etc) if they don't meet up once in a while to be reminded of what each other are really like, they tend to get bitchy and start kicking off with arguments and amateur dramatics. It's fascinating how the internet does that. Cars, too. As soon as people are 'hidden' from each other they act like cave men - but meet up again and everyone is buddies again.
A true long-lasting friendship usually takes regular meet-ups and a decent amount of effort, IMO.
Sorry that ended up a bit long... bored during lunch hour :p
