One of the most fundamental parts of strumming the guitar is proper pick technique. Your guitar pick should be held firmly between your thumb and your index finger. The pressure applied by your thumb will keep the guitar pick from slipping out of your hand. Try to keep your pick at a 90-degree angle to the strings, unless you are trying to use the edges of your pick to create more percussive, scraping sounds. Apply even pressure as your pick touches the strings unless you are looking to accentuate certain notes and strings.
Try using guitar picks of different weights and thickness. Heavy-gauge guitar picks tend to accentuate the bass and treble strings, lowering the volume of mid-range strings. Lighter, thinner picks tend to be a bit more balanced in the sound they produce. They are also more flexible, making them a better choice for very fast strumming patterns. Experiment with different picks when you are playing and see which one works best for the song.
Before you try to tackle complex, difficult strumming patterns, you should master a few basic ones. Strumming your guitar in a downward-then-upward, fluid back-and-forth motion is the first thing you should learn. From there, learn to strum in a repetitive downward motion without strumming from your high strings back up. Once you have learned to play these patterns at a variety of slow and fast tempos, you can begin learning more intricate strumming patterns.
Practicing different strumming patterns without fretting notes or chords can help you by allowing you to focus on your strumming hand without worrying about fretting chords correctly.
When you learn a new song or strumming pattern, slow your strumming hand down below your intended tempo. Doing this will allow your brain and body to remember more information at one time. As you get more comfortable with the strumming pattern, increase your speed. Once you reach the proper tempo of the song, you might learn to play it a bit faster as well. If you are able to do this, then you probably have the pattern and song committed to memory.
It is important to be relaxed when you are strumming your guitar. Players that have stiff arms, shoulders, and strumming hands will sound stiff, rigid, and often amateurish. Your strumming rhythm will sound forced and out of time with the music, and listeners will notice this.