The 10% discount gets you the contract. Now it's upsell time.
"Hey, how about a nice chandelier in the entrance foyer instead of that plain vanilla overhead?"
"You know, recessed lighting would really punch up your kitchen."
"I think we can do better than this Home Depot junk for your front entrance exterior lighting. You do want the house to look classy from the outside, don't you?" [Especially if you've already sold them on #1 and #2!
]
"A lot of guys do some serious woodworking/metal crafting/name-a-hobby in their attached garages. How about we upgrade to 20 amps and add a few spare circuits?"
"Some nice low-voltage lighting would accent your front walk really well."
"The end of your driveway is going to be a little too dark at night. A lamp post would be just the ticket to help spot the driveway apron in the dark with a driving rain and make sure the kids haven't left anything on it they shouldn't. Can't be too safe!"
"We could put some 8' fluorescents in the attic, keep you from misstepping and dropping through the living room ceiling."
Everything of course is cheaper to do "now while the walls are open" or "as long as we're on the job now". Picking jobs that have upsell potential can be tricky, but those are the ones you go in with the 10% discount. They are also the ones were you spell out in fine detail what you are providing. For example, if it's resi work you want to go floor by floor and room by room for switches, receptacles everything. It doesn't have to be a book, an Excel spread sheet with device summaries will do nicely. And signed change orders for each upsell.
Come to think of it, contracts, work summaries and change orders are the way to go for just about any work you do.