Do not assume that your groomsmen know how to tie a tie or bowtie. Ask them learn in advance and practice!
Make a list of everything you will need to bring with you on the big day. Gather everything in advance and put it in one spot so that you don't have to search for an essential item at the last minute.
Remember to factor in taxes and gratuities in your wedding budget. These often are not shown in the vendors' proposals.