1. Choose flowers that are at their peak. Choose bright and colorful. Choose when flowers are at full bloom when their color is at the brightest. Reds and purples will darken considerably when pressed, deepening to a dark red/purple, almost black.
White flowers will turn various shades of cream/yellow. This doesn't mean you can't press them or use them. Just be aware.
2. Prep work. Some flowers, especially larger blooms like roses or peonies need to be thinned before pressing. Pressing the entire flower would most likely mold. Instead, carefully open the flower and pluck some of the petals to press. The petals closest to the center of the flower will be the smallest and will also vary in color. Roses are a great example of this. I recommend pressing many different petals to get a full range of colors and shapes.
3. Don't leave out the small ones! Filler flowers - ferns, Baby's Breath, Queen Anne's Lace are great to press and usually turn out very well.
4. Press enough. Be sure to press enough flowers for me to work with without overcrowding the press. Do not overlap the flowers.
5. Store it well. Once you have your press full of flowers, place it on a shelf or table that is out of the way, dry, and out of direct sunshine - and leave it there for 3 weeks before peeking. Never put the press inside a plastic bag or box. This will cause the flowers inside to mold.