I’ve done it twice, and there has been a lot of good advice here.
As far as people that you need to make sure are paid, it is often a good idea a couple of weeks before the ceremony to sit with the groom (or bride usually, at least in my experience) and have them prepare cheques for those that need to be paid and the right amounts for their services. Keep them in clearly marked envelopes and bring them with you to the service, etc and pay as needed. I generally handled the tip to the minister myself.
You are also basically the gopher. Anything that is needed whether it is a ride, or whatnot, it is up to you to make sure that they get to where they need to go. I delegated groomsmen for that as required. Also black socks are a great idea just in case some people forget about them.
After the groom decides what he wants for tuxes (usually with you) then it is up to you to co-ordinate all the groomsmen getting theirs on time, and sized right. Ride their asses if you have to.
During the ceremony your job is the easiest of all. After ushering for a bit, you get to hang out “backstage” beforehand and joke around lightly with the nervous groom. Then make sure he looks perfect, hair, shoes, boutonniere, etc, and go out and stand up front for the big moment. Be ready to catch him if he falls 
The reception is when you should take a moment to dseek out the parents of both parties and tell them how beautiful / fantastic / wonderful their child is and how perfect this moment is. They will be fretting over all the details still and if you can assuage them even a bit but saying nothing but wonderful things, then all the better.
No puke stories in your speech roasting the groom. You want to be funny, but tasteful, and nothing that will cause the parents on either side to get upset. And never piss off that woman in white on that day. No stories of Thai hookers, for example.
Lastly, after all the toasting, you along with the MC are basically in charge of the party now. After frist dance, boquet / garter toss, it is up to you to make sure everyone is having fun. If you see not a lot of people dancing, get the DJ to play something different.
Lastly, make sure you have a great time yourself. Stay fairly sober, but if you look like you are having fun, everyone else will too.
This is a celebration, so live it up. Just make sure you get all the details taken care of before the party gets into full gear, and that you are there to the end to make sure it wraps up well.
Have a blast. All this is easier than it sounds.