I had some issues with customs, so I thought my experience might be helpful to share.
We took our wedding party gifts with us (9 pairs of Burberry sunglasses). I mis-understood the customs form and did not declare these, as well as all the rest of the wedding stuff that was "not for personal use" (all OOT bag items, chari sashes, favors, etc). You ae allowed to enter Mexico with $300 (per person) of merchandise not for personal use. Everything above that must be declared.
They saw the sunglasses when they scanned our bags and started asking questions, but I didn't know what the issue was, so then they searched ALL our stuff and basically threatened to confiscate it all. We ended up having to pay a 16% tax on the value of the sunglasses (SO GLAD I had the receipts!) and they let us go with the rest of the stuff.
My advice is to declare everything, and have your receipts. And give your wedding party their gifts before you go!
I kept a copy of the declaration form, I will scan it and post it later incase you want to read it before you go.