I always use a bucket with some dishwashing soap. No particular brand, just what the wife has in the kitchen. I don't use much either. Usually about two tbl spoons to a 5gal bucket.
I also use a spray bottle with a little soapy water. It helps to give your panel a quick spray from the bottle if your sanding block is starting to hang up / stick. Hopefully everyone is using a sanding block! The rubber ones work well. I also use a piece of heater hose with the paper wrapped around it for the curved areas. Finally there are foam sanding blocks that work well for tight spots where a regular block, or hose wont work.
To check how the sanding is going I use a soft squegy (sp) I find it works better than using a cloth to wipe away the water. It also makes seeing the low spots / orange peel very easy.
