Lots of good advice so far on here. These are my thoughts, some of which have been mentioned already:
1. The forecast looks to be better than last year, at least as of today. It looks like it will be warmer and less windy. I'm not sure which direction the wind will blow, but last year it was a head wind for a large chunk of the race and it got stronger as we got closer to the finish. It looks like it could rain just as much or more than last year though. But because the other two things aren't as bad I'm guessing we will all be much more comfortable.
2. Know yourself. My wife and I both ran it. She's a tiny girl and I'm a bigger guy. She wore a long sleeve, hat, gloves, and short tights. I wore short shorts and a singlet. I had gloves and a hat on at the start, but I threw the hat off at about 5k and the gloves off at maybe 10k or 15k, can't remember. Both of us prefer colder conditions and I think we wore the right things during the race, but we were both freezing after the race until we got our stuff. Had either of us stopped to walk I'm sure we wouldn't have finished. Instead I ran a 2:51 when I was hoping for about 2:48 (PR is 2:40) and she ran a PR of 3:17, although she was hoping for sub 3:15.
3. Bring throwaway clothes. They didn't limit what we could bring so I brought throwaway shoes, pants, and a sweatshirt and wore a trash bag. I changed into my race shoes maybe 5 minutes before the race and threw off my sweats probably 2 minutes before. I was immediately soaked once I got racing, but I was plenty comfortable at the start and I think that made a big difference mentally.
4. Take your nutrition. I'm bigger so I discovered a while ago I need more nutrition. So I bring several Gu's ( I think I took 5-6 during Boston last year, Boom is my brand of choice) and always try to do Gatorade and water at every station. Just because the weather sucks doesn't mean you should skip these stops. They're still very important.
5. Just race. Forget about time. You've done at least one successful marathon before this. You know how it should feel. I always try to feel bored the first 10 miles of a marathon to make sure I stay within myself. I certainly wasn't bored mentally as it was my first Boston and I was soaking it all in. But I definitely let alot of people pass me (started in wave 1 corral 1). It felt like I was getting passed for at least the first 15k of the race. But then I spent the rest of the race passing people. My splits weren't great. I think I was 1:23-1:28? But the first 5k is especially fast on that course. But I will say that the carnage on the course definitely helped me. Every time I passed somebody who was walking I would use it as motivation. In weather like this I think it's really helpful to use the things around you to keep going. I used the crowd alot too. They were already great, but I would wave my arms to tell them to get louder (yes, I was THAT guy) and it really helped me out. I normally don't do that, but I thought it was appropriate for this race and it seemed to work.
6. Smile. this is not so much my advice but Kipchoge's. I read somewhere that when he starts hurting in a marathon he smiles and it makes him feel better. I tried it for the first time in a marathon and it really helped.