Advice on gnat like insects

[tarpalsfan]

Help and advice please!
Last season some sort of gnat like insects attacked my young plants. The 'gnats' attacked not just tomatoes, but just about every vegetable or flower, especially the seedlings. I used a pyrthium spray to try and stop them with just fair results. I tried dish washing soap water too, and just about killed my plants myself!(Palmolive ) I also spent a lot of time picking them off my plants too.
.
This week we had 3 days of rain, and I drug all my 50+ plants inside, today it was sunny so I took the plants out for some much needed sunshine. The plant sucking gnats are back, swarms of them.
.
Can someone please tell me how to stop these dreadful gnats? The plants are still little, none over 6 inches tall. I don't want to loose my little plants to some darn gnat, I hate it, but will use 7 Dust as a last resort.
.
from Kathryn