If your having trouble switching to the next higher string (IE from D to G), try this exercise to improve your ability. Use a different finger for each note. After completing it once, move one string lower and repeat the same pattern until you are on your lowest string, then go up again. The important issue is not speed, but regularity. Start slowly and play it with a regular rythm. When you can do that correctly, try it faster. You can also use this exercise to improve the agility of your other fingers if you are trying to add another finger to your fingering technique.

Hope this helps :)