I would imagine the main thing is location - passing trade. Then, getting a good reputation, to lure in those who didn't happen to be passing, but don't mind a detour.
There's a cafe in Etal, Northumberland, I visit with friends, the Lavender Tearooms.
http://www.thelavendertearooms.org.uk/
It's also the village post office and shop. Etal has a castle, which attracts a visitors, and it's near a little steam railway, so it's an area with passing trade both on two wheels and on four (and often when we're there, three!). We've been going there for years in groups, and it's not specifically a cyclists cafe, although they installed in the last couple of years a row of wheelbender racks outside. What brings us back, year after year is the cheery service and the good cake, and, to some extent, tradition. So it could be a matter of attracting the right people, serving them well, and waiting for them to tell their mates.
There are plenty of us whimsical cyclists who'd be attracted to somewhere that happened to have a bike shaped plant pot holder in the garden - and if the tea and cake were good, we'd be hooked.
Cheap is also good. Big portions, good value.
The little touches would be a track pump available to borrow, some puncture repair kits, some basic tools.