You need to work on stretching and strengthening your fingers (in that order). An old guitar teacher of mine gave me this little stretching exercise as a starter:

E-1-|---|-3-|---|-4-|---|---|---|---|---|---|---|---
B---|---|---|---|---|---|---|---|---|---|---|---|---
G---|---|---|---|---|---|---|---|---|---|---|---|---
D---|---|---|---|---|---|---|---|---|---|---|---|---
A---|---|---|---|---|---|---|---|---|---|---|---|---
E---|---|---|---|---|---|---|---|---|---|---|---|---

The frets correspond to the frets on the guitar neck. The number on the fret represents the finger you should place on the designated fret. (index finger=1, middle finger=2, ring finger=3, pinky=4.)

Play the above pattern in this fashion starting on the 1st (or high E) string:

1 (hold for 4 beats), 3 (hold for 4 beats), 4(hold for 8 beats). Then back down: 4 (hold for 4 beats), 3(hold for 4 beats) 1 hold for 4 beats. Then repeat this pattern on the 2nd String, 3rd String, ....All the way down to the 6th (Low E) string. Make sure when you play the pattern that you keep your other fingers down on their respective frets (for example, when playing the note on the third fret with your 3rd finger, keep your first finger on the first fret. When playing the note on the 5th fret with your 4th finger, keep your 1st finger on the 1st fret, and your 3rd finger on the 3rd fret.

This exercise really starts to burn, especially as you get closer to the 6th (Low E) string. Keep at it and do it every day. Once you find that you are able to do the above exercise with relative ease, then there are variations of the above exercise which you can use to stretch and strengthen your fingers further. Some of the variations are:

1 - 2 - 4 (top number = finger)
1 - 3 - 5 (bottom # = fret)

1 - 2 - 3
1 - 3 - 5

1 - 3 - 4
1 - 4 - 7

1 - 2 - 4
1 - 4 - 7

1 - 3 - 4
1 - 4 - 8 (I think you get the idea....)