Always have the basics in stock. Set up your eftpos program to have a good re-order level for the obvious stuff… Tubes, patches, co2 carts, energy powders, energy gels.. and the obvious spare parts.. mech hangers for ALL the bikes you sell, bar tape, brake bleed kits, pads..
All the stuff that someone will be frantically needing at the last minute you've got to have. These are the staples and the bits that folks will remember you for.
Don't open Sunday, even for a couple of hours. Pissing off your staff to hang around in a mostly empty shop won't make you friends. Opening late one day a week will net you more money, and cost you less in staff time. If you do open late, post a big sign on the door so people know it.
Advertise. I've worked in some very niche shops who expected people to just walk through the door. Make it directed so you don't waste money. Hitting large local employers with cycle scheme advertisements would be good. That's the kind of thing that generates repeat business and good word of mouth.
After a year or 2 when old stuff starts to accumulate, be ruthless in clearing it out. Open an ebay shop. Don't let the old stuff use up the space you have, and if it ain't shifting then don't fret about selling it at a loss. If you don't sell it now, you'll only have to sell it at a greater loss later. Keep your stock as low as possible while retaining what you want to sell. That way you will stay profitable rather than getting drowned in stock.
In servicing have a clear price structure, and make sure it reflects the real costs of doing a job. Produce service reports on jobs, noting parts that may be due for replacement, and make sure your eftpos program keeps a record of these. Never offer a discount on labour. It's the largest drain on your profit so sell it for what it's worth.