You dont spend the points for a bloodthirster to use him in my experience... he has one of two major roles, you have a big scary bloodthirster painted awesome blood everywhere and you plunk him in the middle of the table (if there is cover, GREAT!) but you make him as scary to your opponent, brag about how powerful he is give him the big damn axe ect... run him up the middle of the table and (unfortunately) wait for him to die... [if he survives, take the opportunity to wreck face!!!!! blood for the blood god!!!]
so what has this achieved? instantly puts the opponent on the deffensive, you on the attack, and protects the rest of your army from a good chunk of your opponents shooting. freeing up your lord on juggernaut with Kor'lath the axe of Ruin, marlerfiends, soul grinders and sea of bloodletters to run up and punch your mate in the face... not literally, on the tabletop.
Option two.. hide him in cover, difficult because how big he is, put him either side of the table and try to make your opponent forget, keep him back, wait for the end game when there are a few space marines in a rhino, troops choices, terminators, tau somethings, eldar somethings and then your bloodthirster jumps out and cuts these down like a hot knife through butter,
hope this helps, and if it doesnt work, well summon him, good luck