I know this isn't strictly about birding but who doesn't like Eastern Bluebirds?
So this spring I put up two bluebird nest boxes (X-Box design on EMT posts) only to have the bluebirds repeatedly evicted by house sparrows.
A few times I ready about using fishing line to deter the house sparrows, and it has actually worked! I attached two horizontal beams to the tops of the houses and then hung monofilament weighted with large hex nuts.
That was about six weeks ago and now we have a bluebird family in one and house wrens in the other.
Edit - on my lappy so I'll post some links
I know a similar method is used to keep birds from grabbing fish in a Koi pond. Something about the filament deters them. The sparrows often land on top of the box first so I wonder if this has the same effect of stopping them from dropping into the filament.