Here is a great exercise to help strengthen your hands and fingers and to build up calluses:

This exercise will also familiarize you with the entire neck of your guitar and give you the agility to play chords and solos. Building the muscles in your hands will improve your stamina for playing and will develop what is called "muscle memory".

The object of this exercise is to play each note going up and down the neck of the guitar. Start with your pointer finger on the 6th string 1st fret, let your fingers follow up the neck of the guitar, one note at a time, staying on the 6th string until your pinky finger reaches the fourth fret.

Play the 1st fret with your pointer finger
Play the 2nd fret with your middle finger
Play the 3rd fret with your ring finger
Play the 4th fret with your pinky finger

Note: After you have played the first fret do not lift your pointer finger up. Keep each finger on the string as you play them. This is important as it will aid in the strengthening process.

Repeat the same process on the 5th string, then the 4th, and so on. When you get to the bottom string, slide your entire hand over one fret without lifting your fingers.

So at this point, your pinky finger should be on the 1st string 5th fret, your ring finger on the 4th fret, middle finger on the 3rd fret and your pointer finger on the 2nd fret. Now play in reverse, starting with your pinky, going back up the guitar to the 6th string, only this time you have to lift up your fingers as you play.

Then simply repeat the process until you reach the last fret of your guitar. Do not rush this exercise or you will develop poor playing habits. Go nice and slow making sure you are producing a nice tone with each note you play.