You do have to be very careful when layering unless you know what you're doing. I happen to like layering sandalwood with leather or tobacco, a very natural and earthy combination for men. I also happen to like vintage Cabochard, a very hard to find women's scent based on leather and chypre (fern, oakmoss, etc) and layer it with some of the more modern incense scents like Comme des Garcons Incense series. The popular gimmicky fragrance "DZING!" also layers well since I think it's too sugary sweet in full concentration on its own, but it adds a nice vanilla/rubber accord. Essential oils also layer nicely with fragrances. Citruses or florals add a nice top note interest to very heavy basenote-rich scents.
Don't mix Cool Water with Brut, or TAG with Aramis, and get rid of the Drakkar Noir from your closet; there usually isn't any chance of disquising discount outdated perfumes and they smell very dated.
