######Preface: I know nothing about your LoL knowledge; if you don't know what any of the language I use in this post is, just ask!
Well, there are a couple ways, and it depends on how the game is going. In general, remember that ~30cs=1 kill.
First, if you're ahead, (well, always really) you should be warding. But I know it's hard to justify warding when you're behind, if you can get one more item and come back with no wards (even though wards are better, I understand the struggle). If you're ahead, though, you can give up that item and still be ahead, so you have no excuse not to ward. Upgrade to a pink warding trinket, and but that in the river brush (circular) on the side closer to dragon. Put a normal ward by the enemy wraiths, and the enemy blue buff. When you see the enemy jungler doing wraiths, ping, so your team knows where he is. When you see him doing blue, go over to take the blue, and/or kill him (if he's low after doing blue). At the least, if he's trying to give blue to your lane opponent, you can force him to smite it (and if he doesn't, you can take it), denying it from your lane opponent).
Additionally:
-
Are your other lanes winning? If so, stay in your lane, get the enemy tower low, but don't take it out. Now, freeze your lane as close to your own tower as you can get it. Stand between the enemy minions and the enemy turret, pushing your lane opponent away. If they fight, kill them. Otherwise, just last hit the minions while they lose out on farm and XP. Come team fights, they'll be useless, and you won't. The reason you take the tower low but not out is that it will prevent your opponent from freezing the lane near his 2nd tower (dangerous for you), but it will be easy to tower dive him for the kill, later (run up, kill the tower, then kill him).
-
Other lane(s) is/are losing, not badly? Take your tower, shove the lane out, then roam to help that lane. Keep shoving and roaming until the lane that was losing, is winning.
-
1 lane is feeding, the other is winning or losing, not badly? Same as #2, but roam to the lane that's not feeding and try to get them fed, also. Roaming to the lane that's feeding is dangerous. Instead, this lets you get 2 fed lanes to their 1 fed lane.
I know these are all suggestions in lane, not in the teamfighting phase, but that one is a lot more champion specific, and depends on how your teammates act. I'll cover it later if you want, but I'd suggest trying these first.