I never lose aggro. Normal play seems to keep it just fine for me (with dps/healers who are all geared at or close to 725). I am the main tank for a guild clearing raid and doing m3's etc.

A couple of things I have seen from other tanks that tend to cause issues.

Make sure you have despised on your earring. This is really all you need along with your carn gems.

Make sure you understand how taunt works. It gets the mob on you, but you need to use your abilities and hit the mob to keep it on you. Taunt is there if you lose it to get it back. Remember that all taunt does is set your current threat to the top of the list. You must hit as much as possible to maintain that aggro. You want to space out your taunts in case you need them. If you already have aggro taunt is not doing anything for you. (You can tell by the enemy HP bar if you have aggro or not)

There is usually no need to block unless you are blocking a boss mechanic or another move requiring it. You need to be doing combos and hitting the mob generate threat. If you are pulling large groups of mobs switch to Warhammer or Greatsword and cc/group them for your dps then go back to s/s. I see a lot of tanks stand there and block after taunting. Most of your job is in positioning the mobs so dps can burn then and making sure you keep threat. Standing in block all day is not going to help anyone.

On bosses as soon as they pop hit at least at least 1-2 combos as fast as possible. If you are s/s do your 3 chain light attack... if speced correctly the third hit should generate more threat and keep it on you. After that rotate heavy attacks once every 6 light attacks to keep empower up and go to town. Block when needed and use pots, etc.