When buying your fabric, please take a little more than needed. (+1 meter or +half a meter) Why? If something is made wrongly or cut wrongly you still can use the same fabric. It is not the first time that you find X-color in X-type of fabric. To find the exact same type and colour again can be sometimes really hard!
If you are unsure of the model cut, make a try-out piece of cheap fabric. It is called a mock-up.
Hoop skirts: if you are tall as me: calculate the cost of making one. If it turns out cheaper on eBay (incl. shipping costs) then I suggest buying it there. Afterwards if you need other hoop skirts (smaller, less wide), you can re-use those existing hoops from that bought one.
When ordering items for your cosplay (fabric, shoes, contact lenses, …) make sure you pay it by paypal. Using a bank account or cheques makes it harder for you to get your money back.
When buying fabric online: please take into account that 1 yard is different than 1 meter (height). The width is in Europe mostly 1,5 meter while in the US it is less large.
Make-up: try outs are good. Do not forget them. You can learn a lot from it. Sometimes adding a little more can be a little different than the original character but more matching with your facial structure.
Always keep: Mastix, Contact lenses fluid, fluid for your eyes when wearing contact lenses, … to you when going to cons. The same for wig headpins, safety pins and threads + needles. You never know when you need them. In my case I even have double sided tape standard in my purse.
Try re-using leftovers, old cosplays you do not wear anymore, daily things you do not need. These things are money savers and better for the environment.