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Getting field testing done, mostly. If you can't/don't field test outside of races, then it makes it a bit more difficult as you have to use other riders as benchmarks, and they're often not consistent (both in power output and also aerodynamics if they're changing stuff too).
Of course I'm obliged to advertise the work I do (currently no open slots for testing though), but you can do it yourself. The simplest thing is just to ride on an out and back stretch of road 1km in length at either constant power or constant speed and see the effect on either power or speed through changes.
If you really want to get faster you find the time to do it - some years back I was going to Birmingham airport at 3am for testing because there was a quiet loop nearby, little wind at that time in the morning and the atmospheric data from the airport was really good.
What does this plugging away involve, specifically?
Genuine question.