From another site. I tried it with very good results.......
Take 2 Niacin Pills (500mg each) about 20mins before you tan. I like to tan after I workout... so before I have my shake, I take 2 tablets, crush them in my mouth with my teeth and swallow them with water. Then, after around 15 minutes you will notice a tingly feeling all over your body (be careful, it gets intense). You will look very flushed - red everywhere... possibly looking like you've been burnt in a tanning bed. Go tan now, and go in for the same amount of time you normally do - and the results will speak for themselves!
Niacin is the vitamin added to all those tingle factor lotions which make you tingle. What happens is that the niacin brings a boatload of blood right up to the skin's surface (thus the flushing), and then the oxygen in the blood will carry the tanning rays even deeper into the skin, resulting in a darker, deeper, more lasting tan without the high cost of the lotions!
I've only done this twice now, and it works like a charm... the results are outstanding. I've gotten about 2-3X darker on each tan doing it with the Niacin... and the best thing is... Niacin only costs about $5 for a bottle of 100 caps!!! That's enough for 50 tans... so its an extra $0.10 per tan to get the tingle factor!!!