How can I stop roast beef drying out?
Depending on your cooking method you shouldn't have any problem with the joint drying out.  By it's very nature, a joint contains a lot of connective tissue.  That connective tissue will gelatinize under a slow roast and make the meat come out extremely juicy.  The best way to do this is going to be a lot of low dry heat (like the kind you get in your oven thats set to about 200-250*F).  You could also slow cook it, as in a crock pot, but only if you don't want slices.  BTW, if you want a crust on that bad boy, just kick your oven up to five hundred when the roast gets to about ten degrees from where you want it, and it should crust up nicely. 

Finally, don't forget that all large pieces of meat are going to keep cooking even after you remove them from the heat so make sure that you take that into consideration when deciding what temp to yank it out at.