Citric Acid, lemon juice, vinnegar, Vodka or Pure Grain Alcohol will help.
Usually adding 5-10% works, but less is more when trying it out. I'd just start with a few drops to a 5ml bottle so you don't wack up your juice too much. If you hate it, it is easier to dump 5ml than 30ml.
Menthol can give you more throat hit as well (if you like it). You can make menthol drops with PGA and menthol crystals. Add that to your favorite liquid the same way. That is what got me over the hump (and I never smoked menthol ciggys, but really liked menthol so used it when I was having cravings for a smoke).
If you liquid gets a bit thin, add a little VG (remember though, adding anything is cutting down your nicotine level a bit). Again, just a few drops to start out with.
A 5v passthrough can help... but then you don't want to be tied to a wire all the time (good for at home). A mod at 4.2v (fresh Li-ion charge) can give a bit better throat hit as well. I find 6v too harsh and flavorless to bother with.