There is a "Just for men" hair thickening shampoo which is rather good.
To make your hair thicker with a thickening shampoo, wash thorougly with a cleansing shampoo like Neutrogena, then wash with the thickening shampoo. Once your hair is squeeky clean, don't condition the thinning area as conditioner usually makes hair look a little thinner. Blow dry your hair and style / gel as required.
Note: Don't just towel dry, as many thickening shampoos use the heat of the dryer to activate and fluff/thicken the hairs, though check the bottle to be sure as some can be towel / air dried.