I am not familiar with white flies, so I am not sure how to address that problem.

Regarding aphids, you can use horticultural soap, or a home made brew of diluted diodegradeable dishwashing liquid. Concentration should be mild - between 1/2 to 1 tablespoon per 5 gallons of water.
Spray the entire plant, paying special attention to spots where aphids gather.
The idea is that the soap (emulsifying agent) breaks down the waxy layer that protects the aphids' body. Once that waxy layer is broken down, the insect dehydrates and dies.
This also has the bonus of breaking down the sugary substance that builds up along aphid hangouts that eventually attract ants.
After a few minutes, rinse the plants with clean water to ensure that no soap residues stay attached to the plants.

Whiteflies love tomatoes. Everytime I have tomatoes in the GH, I get whiteflies.

Whiteflies are pretty sensitive to everything, including soap spray, Neem, pyrethrin, etc. I use a soap spray with baby shampoo (no phytotoxin detergents) at the rate of 1-2T/gal. Spray when cool and the flies are less mobile. You have already discovered that they like the undersides of the leaves, so you have to get underneath everything. Control is possible.

I am about to release 1500 lady bugs in my greenhouse! SHould be fun. I only have a very few aphids, I mainly just want the ladies in there for color.

Had same problem years ago...white flies r a real PAIN.
Found this like ...you might wanna try

Here is a link that might be useful: Homemade whitefly trap

Bihai, I'm just passing through from stained glass/mosaics board . . . noticed your post about ladybugs. Once my sister ordered a huge amount of them through the mail. When she released them, they all immediately flew away. I recently read somewhere that, Before you release ladybugs, mist or spray water in the area. They will stay for moisture, but they will leave if it's not present. Good luck!

I am sure the ladybugs will never want to leave Bihai's greenhouse. It is a flora/fauna paradise :)

Ladybugs are an excellent option though. If they are available locally and your garden can sustain their feeding needs.

I thought of ladybugs too and in a GH, they aren't likely to leave. But I also wonder if some smaller mantis species, like the bright green wouldn't be a good idea too, for checks & balances.

When I was very small child, I was terrified of mantis, until I learned more about them. Marvelous creatures.

Thanks everyone for the great advice. The soap sprays sound like a great solution especially if it works on both aphids and whiteflies. Guess the trick is to catch them early. Last year I didn't notice them until they were all over the tomatoes. I'll try making some sticky traps too.