Nothing brings home the division and chaos of the MO Synod's worship practices like filling in for someone else or having someone else fill in for you. Here are some things to consider, garnered from my own mistakes and troubles in this regard.
First, if you are looking for someone to fill in for you, put together a standard letter that explains in detail anything that might be remotely quirky about your parish. My letter includes the fact that we genuflect, elevate, wear the chasuble, chant everything (except the Preparation), consume all that is consecrated, etc. Do you do announcements before the service? Which general prayers do you use? Etc. If nothing else this is a good exercise for you to take inventory of the customs and ceremonies you follow - and perhaps rethink some of them.
Second, if someone is asking you to fill in - take nothing for granted. Ask questions. For example, the last time I stepped into a pitfall was when I did not discover the use of grape juice until I was preparing the altar at that other parish. I told the elder on duty that we weren't using that at the service that day. To say the least, a sticky situation best avoided. So ask: do you use only wine for the Supper? Do you use only wheaten bread for the Supper? Does the pastor commune everyone? What vestments are worn? About how long should the sermon be? etc. If you feel conscience bound not to do something that this parish does, you want to be able to decline the invitation beforehand. Since I know I feel conscience bound not to use grape juice, and since I know a lot of LCMS parishes do, I should have asked before agreeing to fill in.
It's been said before and it will be said again: you just can't make assumptions. These are the results of unbridled "diversity."
If you find a good fill-in: pay him well!