Also, don't be that lane that pushes before the 15 minute mark, try to stall it if you can. Unless they've already taken a turret then you're free to push and go roaming. By order of importance, top and bot should try to hold their turrets and continue laning as much as possible with. Allowing mid to roam will allow the Jungle and Mid to be top and bottom at any given moment meaning much more ganks and assist/kill gold.
Likewise, if top is a Bruiser/Tank they should be next to push their tower freeing them up to team fight. Otherwise Bot should push next allowing the ADC and Support to roam and be ready for Dragon clears. This is all subjective to the enemies strategy but ultimately allowing mid to freely roam will increase your kills. Once all three lanes have lost a tower remember to COUNTER JUNGLE. Here's where you can really help your team. Once top and Bot are pushed they are free to counter jungle and ward the opponents jungle giving you eyes on the enemy team and jungler and keeping him from farming. The easiest mobs to take are Wraiths and Golems respectively, I find trying to take Wolves puts you in a rough spot with little way to retreat if you get caught. It also allows the ADC to take the opponents red buff, with 2 red buffs on your team you increase your chances of scoring those critical kills.
Remember MAP AWARENESS, Unless you have wards and know where the enemy team is don't try to counter jungle. It's easy to counter jungle but it's just as easy to have a 3v2 or 3v1 fight break out that you weren't ready for.
Edit: Likewise as was mentioned before don't forget to match your build to the team comp. As I play mid some times I find the hardest thing to deal with is building glass cannon and getting focused. If it's a heavy AD team or the ADC is especially fed, push to a Thornmail and Frozen Heart. Those two items alone will guarantee your survival for the most part. I know a lot of us get caught up in trying to do the most damage possible, but survival is paramount. No matter how much damage you do your tanks cannot protect you at all times, and a dead AP/AD Carry is a useless AP/AD carry.
A note on warding: When you're warding and pretty much everyone should be outside of the ADC until late game, try to drop your wards in an area that covers 2-3 entrace/exit paths. Those paths will be the most used and increasing the amount you can see will generally give you a better idea of where the enemy is. Dropping a ward in the middle of the river where no entrace/exit will be see allows them to slip by undetected. But if you move it just a few inches below or above the bushes you can cover more ground and allow for the most vision possible.