Much the same as cookeaa above, only I didn’t have anything to hang a joist from where I wanted my bikes. So I stuck up some shelving brackets and uprights, ran a length of 2×4 along the top and screwed the plastic hooks into this.
It was probably slightly more expensive than cookeaa’s – those brackets are pretty pricey for something so simple and I used 4 as I wanted to be confident they’d hold 6 bikes on my flaky walls. Still, it was cheap, it’s flexible and it’s easily expandable (for the n plus 1).
6 bikes are now stored on 8ft of wall, each accessible without having to move any of the others. Just make sure you’ve got the headroom / wall height. Even with the shelving brackets as high up the wall as I could get them I had to raise my cross beam by 4 inches or so in order to be able to hang my 29er.