Do not allow the costume to look or smell bad. Develop a washing and mending budget, or find a dry cleaning sponsor for the costume. No one likes to shake the hand of a sweaty, smelly, grungy salesperson. Look into sponsors for your mascot. Items may include having a sponsor's patch sewn on the sleeve of the mascot's uniform, on his shoes, hat, pants, etc.
Sponsors can also provide promotional items to further expose the mascot.
Trading cards, posters, buttons, autograph cards with appearance information can be financed by a happy endorser. That way your mascot is happy they get paid and your sponsor gets a feel-good event to be involved in. Add merchandise showing your mascot on the material. Great mascot items include t-shirts, stickers, buttons, photos, ballcaps, stuffed animals, beanie animals, and coloring books. Work on media stunts to increase your mascot's popularity.