I rarely use the horn though unless its to let a good looking guy know he's gorgeous, or to say hello to a mate. 
i find the horn a very effective tool.
i have often been sitting in traffic, 2 lanes about to go across a junction. i've been in the right hand lane, and then as the lights go green the f in front sticks on their right hand indicator and doesn't move.
my simple reaction is thus: i put my finger on the horn until they get out of my way.
if you are going to do this, make sure the finger is down for good. no point wimping out, make your point thats what i say. it stresses the other driver in such a way that they will never forget to indicate again.
i did the same to a mercedes 190 once in the vavler. he cut me to make sure i didn't get past. the alloy went dink on the kerb. i was well pissed as i wasnt even trying to get past.
on went the horn.
follwed him for quarter of a mile before he finally stopped in front of me and got out. he said something, but i had to reply that i couldnt hear him, cause my finger was on the horn, still.
he took off, like he could out accelerate me. and after 1.2 miles he turned off. and i took my finger off the horn.
so, as i say, dont be afraid to make a point using your car horn.
tin
day 2 of 4 of getting drunk each evening. half way there.....