Ive bought a alpine sub amp and box today im just after some ideas for fixing it in the boot to stop it sliding around 
My sub just sits there! Its quite heavy, it never really slides about or move! You could use velcro strips to keep it in place?
The heavier it is, the more damage it will do in a head on if/when it flies into the skull of someone sitting on the back seat!
EVERYTHING should be properly secured for piece of mind. There's lots of holes in the side panels of the boot, for example - but my preferred method is to make a false floor out of MDF which I screw down and bolt the amps to that.
In my last install I took out the spare wheel (got a can of that expandable foam which THANK GOD I never had to use) and bolted my sub into the screw hole left from the wheel.
Sure velcro is an option, as is sticking it down, but I know people who have bolted a metal bar across the back seats and used that. For pity's sake secure your gear in some way though!

Take out the boot carpet, use it as a template on a large sheet of MDF and cut the MDF to form a new layer for the boot floor.
Now cover that floor in some car carpet (Autoleads make some that matches the colour of the OE carpet very well) so it doesn't look horrible. Spray on glue and some staples underneath where the carpet goes under the floor does a good job of this.
Then remove the sub from the box, place the box where you want it, screw through the base of the box into this new MDF floor (a screw in each corner and one in the middle will do) and re-fit the sub.
As the MDF floor is the same size and shape as the boot, it wont move around. And since the box is screwed to that, that wont be moving either
All this will take around 20 minutes if you're handy with a jigsaw.