RNA isolation from Drosophila head
It doesn't seem like a homogenization problem. Nonetheless you can try this:


Suspend the heads in trizol
Homogenize by passing through 30/31G needles (insulin syringe) up to 10-15 times.
Heating a little might help but I think it is unnecessary for these tissues. 


Insulin syringe is much better than the hand held homogenizer. Freeze-thaw 1-2 times if you want (helps in homogenization).

During extraction you can add 5M LiCl (100µl) + Ethanol (1ml) to 400µl aqueous phase (instead of isopropanol), keep at -20⁰C for 1-2h before centrifuging. Alternatively you can add a little RNAse free glycogen to Isopropanol during precipitation.

PS: the liquid Nitrogen trick that rg255 mentioned is for quickly getting heads and avoiding the pain of dissecting each head.