Consume a lot of protein. 1g of protein per pound of your body weight. Don't just count the protein from your shakes, though. Count the protein from other beverages and foods you consume throughout the day. Your body will tell you if you've had way too much protein, as I've heard consuming too much can make you shyt.
Look into some different protein blends, such as Isogainer. Also, search the Recipes forum. Look for homemade weight-gainer recipes, as they're more effective and healthier than store-bought blends.