Practice scales. One of the best ways to increase strength on your fretboard hand is to practice scales. Focus on the pentatonic scale, which is the most basic. Scales will help you gain strength and control in your fretboard fingers and hand. Use a pick while playing scales. As an added bonus, scale practice will help you learn lead guitar. The pentatonic scale provides the foundation for a large portion of lead guitar solos.
Practice chords arpeggio. To increase strength and coordination in your strumming hand, play the chord arpeggio instead of with a strum. Arpeggio means to play the notes individually instead of simultaneously. With the lower palm of your strumming hand resting lightly on the guitar, pluck each string in the chord with your thumb and fingers. Do not use more than one finger at a time and begin at the lowest note in the chord and pluck to the highest note. Then go back down again using the same fingers for each note. Repeat.
Do chromatic exercises. Chromatic exercise is a technique where no particular chords or scales are being played, just notes in sequential order. Using your fretboard hand, begin on the low E string, on the first fret, and move up the fretboard with each finger. When you get to your pinky, repeat the exercise on the remaining strings. When you get to the high E string, reverse the exercise by beginning with your pinky finger and work down to your index finger. When you arrive at the low E string, move up one fret and repeat the exercise until you get all the way to the end of the fretboard. Use a pick for chromatic exercises.
Squeeze a stress ball. When you’re not near your guitar, try squeezing a stress ball with your fingers. There’s no need to squeeze with your hands because squeezing with your fingers will strengthen your hands as well.