Put in cards that draw extra cards--this will help you find what you need, when you need it. Cards that do an effect and then let you draw are especially good for 60+ formats. Here are Instants and Sorceries that help you draw.

Include cards that search for lands, since 60+ decks are often multicolor out of necessity. Green is especially good at land search, but there are plenty of other cards that help you find the land you need--here is a list of all the cards that search for a basic land. Nonbasic lands that you sacrifice to find a basic land will also be your friend in a 60+ deck--see a list of all of those lands here.

Also, you can add in spells that can search for other cards besides land--there is an older set of them collectively called the "Tutors" (which is why the "card-search" effect is often called "Tutoring for a card"). See a list of all the "tutor" cards in the game here. These cards help you get the other spells you need right when you need them. There are a lot more Tutors in Black than in any other color, so keep this in mind while you build. Also, some of the Tutors are restricted (you can only run one in a deck) or even banned in certain M:TG tournament formats, so build for the format you'd like to play the deck in most often.

(Important Note: some of these Tutors are instants, you can play them at the end of your opponent's turn. For the Tutors that make you put the searched card on top of the deck, it's a nice way to draw exactly what you need next turn.)