Planning weddings is a little bit like a game of wack-a-mole: you tackle one big detail - like your venue - and suddenly a bunch of other details that need to be resolved, rear their heads. The venue now needs you to get vendor certificates of insurance, and deliver a timeline and diagram; you need to hire a florist; your tall bridesmaid can't fit in the dress that everyone else likes and is on sale for just one more week; etc etc. And don't get me started about all those wedding checklists out there on the world wide web - there are so many, and some of them are exhaustive (and exhausting). Having a system to stay organized is the best way to keep productive and sane.
Get a Binder and use GoogleDocs.
Binders are the best - you can print out and store contracts, timelines, and all other paperwork in divided sections. Also, start storing your guest list on googledocs so you can update it at anywhere at any time.
Find a good checklist.
There are a few good checklists out there - Aisleplanner.com is a web-based professional-grade one worth investing in, and serves the same role as googledocs, but like, on steroids.
But, there are a few online wedding planning checklists out there that are over the top. I think they are written by folks who are into providing content as much as they are in providing an efficient framework. My checklist is just one page - the broad strokes - and keeps it fairly simple. Then my clients don't get buried in minutae. This one by Real Simple is very clear, non-overwhelming, and timed fairly well.
Send your fiance to research cake and invitations; your mom to order and assemble the favors. Delegate a few small things so you can focus on the bigger elements; you'll be grateful for the help!