Do as well as you can on your preclinical classes, kick a$$ on step 1 (i'd say 230 absolute minimum, 240+ is more realistic), get involved in derm research early in your med school years. Try to pull down some honors in 3rd year and possibly get AOA, smooch some derm attendings' rear for a good letter, do a couple subI's and really kick ass in them, and pray to your higher being and hope that she will smile upon you on match day.
That's a lot, and it's easier said than done. Most importantly, do your best and try to have fun in doing it. See if you like derm best, or there is somthing you like better.
Some derm residencies have a "research year" option in which you can do a year or two of research which may guarantee you a spot in that program. If you are unsuccessful in matching, and really want to do derm as your career, it might be an option to look into. Not many places have this, though.