Bottles. Ethyl Maltol. More bottles. Sour (Malic acid). More bottles. Citric acid (tart, but not all that sour). More bottles. Menthol, koolada, perhaps some mints. More bottles. White vinegar...sometimes helps. If you're into tobacco flavors, Tobacco Absolute blend. Use these sparingly until you get a feel for them. For my testing, I dilute them 10-to-1, so 1 drop of diluted in 1ml recipe would equate to 1 drop full strength in 10ml. There's 8 bottles just for my diluted flavor enhancing.
3ml pipettes (got mine from amazon.com) used for measuring, or just as a dropper. Plastic tray for mixing station to contain any small spills ( kid out-grew the multi-section plate that keeps one food from touching another). I have dropper bottles pre-mixed with 0-nic PG/VG at desired ratio for testing, and another of PG/VG+desired nic level. Latex or nitrile gloves, for when you're handling the nicotine.
Did I mention bottles? Small bottles/vials for testing. I use a variant of this
http://www.e-cigarette-forum.com/forum/blogs/str8v8ping/2705-diy-tips-str8.html for my testing, at 0-nic, in 1ml samples. Remember to start with small amounts of flavoring, and increase as to your taste. When I think I'm ready for a first batch, I do 3-5ml with nic, and adjust flavoring to taste.
Label your bottles. I print out on laser printer now, but used to do pen/paper. Small enough label to cover with clear tape. My tape is 3/4 inch (18mm) wide, so my slips of paper are 1/2 inch (12mm). Smaller bottles are labeled '1', '2', '3'...to correspond with my notebook. Keep good notes as you go.