I have been costuming for around eight years now and these are some of the things I have learned.
1) Don't sew when sleepy or you will sew things to yourself.(blood is hard to get out of white fur)
2) Bias tape makers are worth their weight in gold.
3) Irons need a good bath now and then too. (Hot iron cleaner is lovely stuff.)
4) Make sure you have all the pins out before wearing something.
5) Label your mock ups well it will save you a lot of time. (Sewing things upside down and then figuring out why they don't fit later is embarrassing)
6) Make absolutely sure you are cutting the pieces the right way up.(Midnight fabric runs because you messed up and are hitting a deadline are not fun.)
7) Remember your seam allowances they are the difference between having a fantastic fit and a lovely thing that is too small.
8) Take things one step at a time ot you may find yourself skipping important steps. (like trying to do the bust darts after sewing the sides together)
9) Pre-wash and iron your fabric. Having your new lovely red shirt turn you a matching shade is not fun.
10) Buy extra fabric when you can. It may just save your soul. (Or your sanity whichever comes first.)