When should I trade into opposite colored bishops? This is something that always confuses me. If I have an edge, I may trade and the game becomes drawish. If I am down, I seem to trade and I get ground down.

I'm aware of things like potential weaknesses or space to consider, and I know that putting pawns on the same colored square as your bishop should be avoided.

But how do you evaluate if the advantage is enough? And what does give advantage e.g. what pieces left on the board are better for the attacker? What sort of checklists are there? I know that for R&amp;P endgames, there is an active rook and so forth, but with opposite colored bishops it seems less clear.

This is a separate question from actual technique. I just know I avoid trades to opposite colored bishops on principle, because I'm clueless about them, and that's a big problem. And while it's fun to see GMs capitalize, I still don't have an idea of the thought that goes into when to trade.
Opposite-colored bishop endgames aren't always drawn; it depends on the pawns. The drawish aspect comes from the fact that either bishop can halt a passed pawn in its tracks, and the opposite bishop can't help overcome the barricade. 

If you have a positional or material advantage, avoid going into such an OCB ending if you can. If you can't, then start action on both sides of the board, creating a passed pawn on each. Sac a pawn to accomplish this, if necessary. This will force your opponent to dedicate his bishop to one side and his king to the other.  

Leave your bishop to guard one wing, preferably the wing where the opponent's pawns are less of a threat, and/or yours are easier for the bishop to defend. Then, send your king over to help push the passed pawn (or create one). He can chase the opponent's bishop away, or force it to give itself up for the passed pawn your king, but your bishop can't. So, king on offense, bishop on defense. 

Also, try to keep your king in front of the pawns, so he can cover their advance squares.

Note: The OCB ending is an exception to the endgame rule of having your pawns on squares of the opposite color to your bishop. The exception applies only to the defender. If you follow that rule, your opponent's bishop will pick them off, and your bishop won't be able to defend them. So, if you're defending an OCB ending, put your pawns on the same color squares as your bishop.

If you're the attacker, put your pawns on squares of the opposite color to your bishop. That way, they cooperate with the bishop to keep the opponent's king away or drive him off.

If you're defending an OCB ending, keep your king on the same wing as, and in front of, the opponent's pawn majority, and defend your (possibly weak) pawns on the other wing with your bishop. Blockade your opponent's passed pawn with your king. Put your king on a square of your bishop's color, so your opponent's bishop can't dislodge it.

If you can, try to leave the base pawns of your pawn chains where your bishop can reach all of them quickly. If you can arrange two chains, one on each wing, with the bases near the center, then your bishop may be able to defend both from just one square.  If your opponent's king comes over to try to drive your bishop off Defense Junction, then he can't reach the pawns. No way to make progress.

If you can't arrange this, then try to stagger the chains a bit so that your bishop and king can navigate across the board, and keep an eye on the pawns at the bases of the chains. Leaving a few gap squares is important for the king's mobility, because if you don't stagger the chains and your king has to use the opposite colored squares to cross the board, your opponent's bishop can usually just attack the squares around the pawns when your king tries to get through, and block it.

Move a pawn only when absolutely necessary.  If he can't make progress, you'll get your draw.

Almost every endgame book treats this topic; I recommend a couple, such as Howell's Essential Chess Endings, Glenn Flear's Starting Out: Minor Piece Endgames, or Bruce Pandolfini's Endgame Course. Andrew Soltis' GM Secrets: Endings has an especially understandable and thorough discussion of "Bees of Opps".

So, the short answer is: Trade into an Opposite-Colored Bishops ending if you're likely to lose otherwise, and use its special characteristics to save the draw. This can sometimes work even if the opponent has 3 connected passed pawns, and you have none!