1) Make sure you have good posture, as it's most likely your back that will be gracing your book cover. Tattoos would be a nice touch.
2) If you are beaten, mugged, murdered or otherwise assaulted do not be surprised if you wake up in the hospital with hitherto unrealized supernatural powers.
3) You will have no girlfriends. All the women you meet will either be a) passing acquaintances, b) bitter enemies or c) so powerful, you have no common point of reference. If you do have a girlfriend, she'll shortly be leaving to start a UF series of her own or worse see #10.
4) Do not be judgmental about the sexual practices of others. Chances are that at some point you'll find yourself:- a) with a harem or b) engaging in sado-masochistic acts with a demon. And no one likes to hear the words - "I told you so."
5) That vampire/werewolf/weird creature that you can't stand, but are strangely attracted to in Book 1 will be your husband/partner/significant other by Book 3.
6) You are not always right. You do not always know what is best for others. Decisions made when you think these thoughts are almost always destined to end badly. (This also includes the times you tell your allies that you aren't going to do anything, but are secretly planning to follow your own ideas anyway.)
7) If your vampire/werewolf/weird creature (see #5) has an ex-wife, either deceased or just out of the picture. He will NOT have gotten over her. Even if he says he has, there will be some sort of unresolved issue.
8) Be prepared because said wife will undoubtedly make a reappearance at some point in the series, and it won't be to wish you joy in your new relationship.
9) If you have not developed supernatural powers by your 24th birthday. Do not worry. Bestowal of said gifts often coincides with your 25th year and may not necessarily involve being beaten, mugged or murdered. (Though that isn't guaranteed.)
10) If you have a human boyfriend in Book 1. He will probably end up resenting you and running off with the woman you thought was your best friend. (see #3). Or become suspicious and resentful because you never tell him the truth and keep secrets. He'll decide to give you some space and you'll never see him again. Though he may miraculously reappear at the same time as the ex-wife (see #8).
Anyone got any others to add?