There are exactly three levers on margin: what you charge, what it costs you, and which work you take. Everything else is a version of one of those.
Price is the fastest and largest. It is also the one owners try last.
Check the arithmetic first
Before changing anything, confirm you are not losing margin to a calculation.
A 40% markup produces about a 29% margin. If you have been marking up by your target margin figure, you have been under-earning on every job, consistently, and nothing about the business looks wrong. See markup and margin.
This one costs nothing to fix and is worth checking before any harder work.
Price
Raise prices. A 5% increase on the same volume goes almost entirely to profit, because the cost of doing the work has not changed. Very few customers leave over 5%, and the ones who do are usually your worst-paying.
Stop discounting by habit. A 10% discount at a 30% margin gives away a third of your profit on that job.
Charge for what you absorb. Trip charges, after-hours work, extras added on site. Each feels small and each happens constantly — see change orders.
Raise your minimum. Small jobs carry the same overhead of travel and admin as large ones.
Cost
Cost your jobs properly first. You cannot fix what you cannot see, and a job missing its labour looks profitable. See job costs.
Review supplier pricing annually. Prices drift up and nobody renegotiates.
Cut rework. A job needing a second visit usually has no margin left at all — see multi-visit rate.
Reduce drive time. It is labour cost that appears on no invoice. Clustering work geographically is free and is the largest single lever on daily profitability in a service business.
Mix
Find out which work actually pays. The uncomfortable and common finding is that the work filling the schedule is not the work carrying the profit. See job margins.
Do more of the profitable kind by changing what you advertise, which costs nothing beyond the decision.
Price the weak kind higher rather than dropping it. Being busy is not the objective.
Order
Arithmetic, then mix, then price, then cost. The first is free, the second is a decision, the third is a conversation, and the fourth is the slowest and the one most people start with.
Typical results
Margin moves slowly and permanently. A five-point improvement over two quarters is a very good outcome and changes what the business can survive — see growth capacity.