If I wash thoroughly with rubbing alcohol after sex, will I lessen my chance for an std?

No. Alcohol is not a good disinfectant, and burns mucous membranes. Wearing a condom is the only way to lessen your chances for stds. If you wish to wash in addition, use 1/2 strength hydrogen peroxide or betadine, but good old soap and warm water are probably equal. Again, if you are having sex with multiple partners or with at-risk people, use condoms always and throughout the sexual encounter.
No. No, rubbing down with rubbing alcohol is not effective at reducing your risk of obtaining an std. Stds are spread by contact with skin to infected skin or infected secretions from someone who has the std. After you have sex with someone who is infected is precisely the wrong time to be concerned for getting an std. Practice safe sex - always. Protect yourself and be prepared for safe sex!