I found out on the weekend that you need to put a thin coat of buttercream on the cake first (let it set) and then pour the ganche. I found that gave me nice smooth sides (used a knife to smooth out the sides). You also need to pour a lot of ganche to have it drizzle down the sides (it tends to pool on the top of the cake). Make sure also that the ganache is pourable consistency and isn't too thick.
I pour it into the centre and use a spatula to push it towards the edges so that it spills over down the sides. You do need to make enough to get this effect and yes, it does need to be warm enough to still be pouring consistency.
So can anyone tell me what's the secret to completely covering a cake in ganache? If you want it all over the sides, but not on your board, you'd have to pour the ganache while the cake is on a different surface and then move it onto the board, but how is that done without cracking the ganache?
I put it on the base I want (ie. 6" cake-6"board) and place the cake and board on a cooling tray with a cookie sheet underneath to catch the extra ganache.
I pour the ganche on the cake/smooth out the sides etc and then just move the cake (board and all to the final surface).
If the ganche is set enough you should be okay and not mess up/crack the ganche.
The way I do it is to let some ganache set up a bit so that you can use it like a frosting.. give your cake a crumb coat with the ganache that has set up. You might need to warm up the ganache you are using to pour on the cake to get it to pouring consistency if it has set up a bit. After it is the right consistency pour it on the cake like the PP said.. on a s cake board that JUST fits the cake and over a cookie sheet with a cooling rack on top. You'll have to wiggle the cake around to get the ganache to go all over and to prevent getting a really thick layer on the top. Line the cookie sheet with parchment paper so you can easily pick up the leftovers from the cookie sheet to use again. Hope this helps. 
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