a) setting up a git repo with working code in it (including tests, dependencies and whatever else) so I don't have to worry about inane(but taxing) decisions in terms of project structure/code style. Also, verify it works before sending it out to candidates.
b) including clear & detailed instructions on what is expected
c) given the test to current employees (blind) & timed them on it (allowing for the fact they know the domain & technology)
d) make it somewhat representative of day to day. i.e. I have 0 interest in spending time configuring and doing anything which is a 1-off task such as authentication etc..(unless the companies primary product is security/auth)
In short, make it easy for me to prove myself.