Spot the Most Distinctive Features
When you're drawing an object or a personality, spot their most disctinctive features and try to depict it clearly - for example, if your drawing MR T, indentify his prominent features like his mohawk and his thick chains, you can draw it on a stick man and add some muscles to him for good measure, you're friend ought to know what you're trying to get at.
Using a Stylus
Admit it, your fingers obscure the screen and they don't make for very good drawing instruments. This is where a you might want to consider investing in a stylus - it allows you to draw much more precisely and it feels much more natural. A good stylus should not cost you more than $10 and it will come in handy for other apps too, besides Draw Something.
More colors more flexibility
The first things that you should buy in the game would be more colors, they make the words a lot easier to depict and the game a whole lot easier to play. The colors in the game cost coins, and they're not very cheap, I'd recommend that you get colors that you'll need to draw common objects like grapes or rainbows, in other words, get the ones with the larger color palette!