Hello,
To address the question that you have raised here, I should first say that I only have very limited experience with assays using small molecules. I'd be very glad to share the experience with you though.
Typically, I'd add the extracts/small molecules onto the plate to a final concentration of 50uM to 100uM. The cuticles of worms, as well as the chemical properties of the small molecules, have very strong impact on the final bioavailability of the chemical agents that you will test. I'd say that 100uM is probably a good start point, as the final concentration inside the worms could be less than 1uM. You may have to test a range of doses though. I'm not sure what kind of assay you will be doing. In the case of lifespan assays, it'd be better to start the treatment at adult stage to avoid potential effects during development.
You can add the extracts/small molecules directly onto NGM plates, or supplement it to the medium. Make sure that high temperature won't cause issues of degradation/oxidation, etc. Try to prepare and use fresh plates, and cover plates with aluminum foil to protect them from light --- it should be OK to store them at 4C for short term (high level of DMSO may form crystals though).
I'd prefer to use salts to get better solubility, and particularly, for hydrophobic stuff such as lipids. Tergitol (Sigma, 0.1% final) can be used to help dissolve lipids on plate as well. Should DMSO be used as the solvent, DMSO only should be added to the control plates as well. Try to control the final level of DMSO to be less than 0.5%. In this case, it'd be better to prepare a stock solution of more than 100mM (DMSO final concentration would be 0.1% after 1000X dilution).
A 2010 BBRC paper from a group in China showed that DMSO (0.5% to 2.0% final) actually extends lifespan in a sir-2.1- and daf-16-dependent manner.
Lifespan extension in Caenorhabditis elegans by DMSO is dependent on sir-2.1 and daf-16.
http://www.ncbi.nlm.nih.gov/pubmed/20828537
Here is some good references that would be helpful to your question:
1. 2006 Aging Cell paper from Cathy Wolkow's lab at NIA/NIH
Blueberry polyphenols increase lifespan and thermotolerance in Caenorhabditis elegans.
http://www.ncbi.nlm.nih.gov/pubmed/16441844
2. 2007 Nature paper from Linda Buck' lab at Fred Hutchinson
An antidepressant that extends lifespan in adult Caenorhabditis elegans.
http://www.ncbi.nlm.nih.gov/pubmed/18033297
2009 Ann N Y Acad Sci from the same lab about screen
A high-throughput screen for chemicals that increase the lifespan of Caenorhabditis elegans.
3. 2010 PLoS One paper from a German group
Antidepressants of the serotonin-antagonist type increase body fat and decrease lifespan of adult Caenorhabditis elegans.
http://www.ncbi.nlm.nih.gov/pubmed/19112515
Please note that this group reached a conclusion that is exactly the opposite to what the Buck lab found for the effects of serotonin antagonists on lifespan. The major reason, they believe, could be that they used liquid instead of solid plate.
4. 2010 PLoS One paper from Monica Driscoll's lab at Rutgers
Metformin induces a dietary restriction-like state and the oxidative stress response to extend C. elegans Healthspan via AMPK, LKB1, and SKN-1.
http://www.ncbi.nlm.nih.gov/pubmed/20090912
5. 2010 Nat Chem Biol paper from Peter Roy's group in Canada
A predictive model for drug bioaccumulation and bioactivity in Caenorhabditis elegans.
http://www.ncbi.nlm.nih.gov/pubmed/20512140
Best,
Peichuan