I love my best friend

Do you tell them, or do you keep it quiet? The choice is yours.
Maybe you've always secretly fancied them, or perhaps love has blossomed out of a long-term friendship. How do you find out whether they fancy you too, and should you even try in the first place?

Opposite sex
Think long and hard before you make your feelings known to your friend. Be absolutely certain that you want to take the relationship to another level, make sure it isn't a passing crush. Having a quick fling is quite likely to damage the friendship - you could be better off having a fling with someone else you like just as much, and not losing your friend.

If you're sure that it's definitely a big lurve thang, is your friend single? If they're in a happy relationship already then it is probably unfair to try to split them up. If your heart is set on them, you'll just have to wait for it to come to a natural end, and be there after they've picked up the pieces. Just don't start slagging their partner off or interfering in any other way, no matter how tempting it may be. Of course they might not split up, in which case you have to be realistic, and move on to another love interest.

Should your friend be single then start flirting, and look for signs that they may be flirting with you too, remembering that there's no such thing as a 100% definite sign, unless they start snogging you. Be wary of using the most common ploy, which is getting horrendously drunk and either grabbing them and slobbering everywhere, or droning on about how you've always loved them and how you should get married and have babies together. This is quite likely to make them run away screaming, even if they fancied you a bit in the first place.

Be brave, find a quiet moment, and ask them if they want to go out with you sometime. Yes, as more-than-friends. Let them know there's no pressure, and if they say 'no' then you still want to be mates. Be prepared for a knock back, but hope for the best.

Same sex
If your friend is out already, you face the same problems as straight couples if things go wrong: embarrassment, and losing a good friend. Should things go your way, the rewards are the same too: a lover you know you already get on really well with. If your mate is not openly gay, and they turn you down then be wary of a homophobic reaction. Most people are fortunately not like this, but it doesn't hurt to think ahead.

If they say no
Be nice, no matter how disappointed you feel. You may have built things up in your own head, but they just didn't see things in that way. Make an effort to keep in touch, but don't get too hung up about the great love that never was. Chalk it down to experience. Keep them as a friend, but make sure your life doesn't revolve around them, have fun with other people and try to find someone else that you fancy.

If they say yes
Lucky you. Go for it.