Well it helps if you have really good DM since most of the time you are playing away from the combo and you won't be getting heals. You need to play smart. Look for openings to get behind the other team and do lots of damage for your team. Be sneaky and do things that the other team won't expect.
You need to realize that you are totally expendable, and you are the least important person (especially to your medic), and you are going to die a lot. Yes the basic job for the roamer is to watch the flank, get picks, force ubers, make openings, and cause distractions, but what really sets good roamers apart is game sense. Knowing how to position yourself, and when/where to go for those big plays (and when to fall back and stay alive) is very important.
An example of an avarage roamer vs. a good roamer: granary mid... Your team lost the fight and are backing out into yard. You are caught behind the enemy team. They are stacking the point and you know you have to make a play. The avarage roamer might try to bomb the point while they are clustered on it. The roamer MAY force an uber there or even get a pick, but most teams are going to be watching behind them as they cap a point expecting that bomb. The good roamer may very well let them cap and wait for them to let their guard down. When the enemy team is starting to engage yours in the spam fight in the yard, the good roamer makes his move while they are spread out, distracted, taking some damage, and have let their guard down from behind.
Game sense and DM.