When I was practising for a concert playing the Beethoven Mandolin Sonatas I was having difficulty playing a fast quaver passage evenly. This came up with one of my pupils last week and I remembered a very good tip I was given by the pianist I was rehearsing with and it was something that had helped him. He suggested I slow down and play the passage, placing emphasis on the weak beat, in other words the second quaver in each pair. When you do this it takes the pressure off the strong beat and creates space around the first quaver. Secondly it makes you pay attention to playing the second quaver and giving it more than equal importance. Amazing things happen when you do this. Your perspective of the melody changes, you notice the control required for the up stroke and how much control you gain over string crossing and finger placement. The natural lightness of the upstroke is replaced with a power and force turning the expected into the unusual. This reversing of flow and emphasis highlights the parts of the passage that require the most attention and solves quite a few problems along the way.
The power of this strategy is only noticed when you then play the passage at speed. Suddenly I discovered the quavers were flowing evenly and effortlessly because I was no longer throwing away the second quavers. Instead I was really playing them with equal importance and emphasis.
I now do this anytime I find a quaver passage not quite flowing as I would like it to.
Let me know if you find this useful and any other suggestions you might want to add.