Cook up lemon/sugar in your kitchen. Pour some sugar into a small, heavy iron pan.  Pour in just enough lemon sugar to make the sugar wet.  Heat this till its clear and bubbly.  Let it cool.

Paint  lemon sugar syrup onto completely dry henna with a cotton swab.  Be very careful to not disturb the henna pattern.

Blow dry the lemon/sugar glaze so it dries quickly. 
You can also make a rosewater/sugar seal for your henna, or an orange flower water/sugar seal.  Just use rosewater or orange flower water as you would lemon juice to make the syrup.
The drawbacks of these sugar seals are that in very humid weather, they stay forever sticky, and if you're working an outdoor festival, the sugar will attract bees, stinging wasps and yellowjackets.  If your henna table is swarming with bees and stinging insects, it will not be a happy place. You may want to choose a different seal if you're working outdoors.

Want to know more about seals, what people use and how they use them?  Ask the henna artists on the Henna Page Forum! 

Henna Page Artists have favorite Lemon/Sugar mixes and favorite ways of applying Lemon/Sugar:

Bellanoir: I make a simple syrup solution of 1 c sugar, 1/4 c lemon juice, 1 TBSP honey, and cloves heated til it is a little thicker and less likely to drip about consitancy of maple syrup. I usually use cotton balls for LS and glitter gloop. I have also used Natural Instincts spray hair gel, New Skin, and a Mexican Tamarind liquid candy that has a nice little applicator tip to secure henna designs.