many people attach it to a piece of slate (to add the extra weight)...
but if it is properly cleaned (boiled for an hour or so, brushed down real well to rid of the excess debri on it) and soaked to rid of the tannins (causes water to have that dark coloring) and is soaked for some time (generally up to two weeks) then you shouldn't have a problem with it staying in place.
but your best bet is to use a piece of slate and attach your driftwood to that, many LFS (or LPS for the UK) have driftwood for sale that already is attached to the slate
at the same time, be careful using driftwood found in saltwater in a freshwater tank, make sure to clean it real good and boil it down to kill any bacteria or living organisms that may be on it (don't want anything to pop up out of no where)
if you feel neccessary, you can add, i say..., 1 part bleach to 20 parts water and give the wood a good soak there, just be sure to rinse it very well... when i did this, i let it soak over night, and then the next day i left the driftwood in a bucket and let water run through it (creating a constant overflow exchange of water) so that new water kept pushing the other water out, and then boiled it once more and let it cool over night in room temp water (or if you have an extra heating, water the same temperature as the tank it is going in) and then voila...
you have driftwood ready to go
(i've done a bit of research, to say the least... lol )