I can partly guess the feel of shop you're going for from your Sargent example, and know of a few shops in London and LA that already have a similarly relaxed vibe - Tokyo Fixed, Look Mum No Hands, Golden Saddle Cyclery and Orange 20.
These shops capitalise on being within a buzzing community - being central to local cycling events, organising or attending races (though not in an EPO-mainlining wannabe Pro way) and generally doing fun stuff.
They have to work hard to keep their image fresh, and invite like-minded folks to collaborate with them on nice little road-cycling trinkets like hats, tees, bottles and musettes.
Sounds a bit cheesy I know, but as many of the retro-centric cycling brands around today have shown, you can move forward whilst practically standing still.
Beware of making the shop look like a museum. Put too many things high up on the wall, or behind glass, and nobody will buy them. It's totally fine (cool, even) to have a few 'Not For Sale' bits stuck up, especially memorabilia or old jerseys, but the stuff you want to sell should be clearly within the customer's grasp, or marked with a price if it's in a display case.
Personally, I effing hate it when display items have no price on them, particularly with NOS - there's often the sneaking suspicion that the shopkeeper is going to make up the price based on the customer's accent, appearance, etc. It happens.
You likely know about some of these already, but here they are anyway.
Definitely worth setting up a Tumblr and/or Instagram account for your own shop too.
Nostalgia feeds off imagery, just check out http://pedalarepedalare.tumblr.com/
for evidence of that.
You could also entertain the idea (if you like) of doing custom retro builds for clients rather like Andy White of Fyxomatosis