Once henna paste dries, it tends to crack and crumble off, however henna must remain on the skin for 6 - 8 hours for the darkest and longest lasting stain. In order to help prevent the paste from crumbling off, try sealing the henna to the skin with a lemon/sugar mixture.
If you are going to sleep with the paste on, seal it with with lemon/sugar, let the lemon/sugar dry, and then wrap it in a tensor bandage, sock or something else breathable (that you don't mind getting stained). This a good way to prevent crumbs from falling off in the bed, and to help keep the henna warm, which will help darken the stain.
Check out Frequently Asked Questions for more information on sealing/wrapping, getting dark stains, and more.
The following recipe is for one hand or small amounts of henna.
Double for larger coverage.
1/8 cup lemon juice
1/2 tsp sugar
Gently heat the sugar in the lemon juice on med/low just until the sugar dissolves.
Remove from heat and let cool.
Make sure the henna paste is completely dry. It should no longer look shiny or feel soft, and it should be starting to crack and come away from the skin slightly.
Dip a 100% cotton ball into the mixture, squeeze out excess, and gently dab it onto the dry henna paste. Alternatively you can put it into a small spray bottle that will mist when sprayed.
Do not saturate the henna otherwise it will start to melt, you want to cover it lightly.
Allow to dry.
You can apply the sealant approximately every hour, or when you notice cracking and crumbling.
Lemon/sugar can be applied when the henna is dry and starting to crack.