The only trick I know is to put the DME in the flask before you add the water. As you probably already know, you want the OG around 1.030 to 1.040. You do not want to make a high gravity starter to grow yeast. As a ballpark measurement, use about 6 ounces (by weight) of DME to 2 quarts of water. If you're working in metric, it couldn't be easier. Use a 10 to 1 ratio. Add 1 gram of DME for every 10 ml of final volume. (If you're making a 2 liter of starter add 200 grams of DME to the flask, then fill the flask with water until you have 2 liters total.)
FWIW, LME dissolves more readily in hot water and DME in cold water. DME tends to "clump" when added to hot water. Also, I use one of those thin plastic cutting mats to weigh out the DME on my scale and then fold it a little to pour the DME into the flask.