As much as I'm looking forward to ME3, I doubt I'll be grabbing it on release day. Purely coz I don't have the money for it right now. I know for a fact that I WON'T be transferring my file of ME2 over to continue in #3 coz I ended up losing 4 characters in the final battle with the Reapers! I was horrified! Don't ask me what I did or didn't do but despite clearing all missions (main and side), gaining everyone's loyalty, kitting out the Normandy to max spec, I lost Garrus (my fav side character), Mordin, Grunt and Tali. It didn't even explain why Tali and Mordin died. They were just dead at one point!! Clearly I made some questionable choices throughout the game which lead up to their demise. So, in light of that fact, I'll be playing ME3 with a fresh new Shepard. 
I think I figured it out. You not only have to build up the Ship and Weapons, as well as gaining loyalty, you must also build a strong relationship. In other words, you must select the option "I want to know more about you" when you talk to them in their quarters up to the point where they don't want to give anymore to you.
Also, do certain loyalty missions last as well as doing as much DLC missions as possible (ie Lair of the Shadow Broker) to get as much Paragon/Renegade points as possible.
By building strong relationships, you keep the strong Paragon/Renegade option open when conflict moments happen between the crew. I recently made the mistake of not building a strong relationship with Miranda. As such, despite my Paragon meter being full at the end of Jack's Loyalty Mission, I had to choose between the two. The strong Paragon choice was not open, sadly. As such, I chose to keep a strong relationship with Jack. This in turn helped keep them both alive through the final mission.
Finally, do not use ANY DLC characters for any of the options in the final mission. They only gain loyalty to you and NOT a strong relationship. As such, you risk having them die if you put them in charge of a team or certain suicide projects.
By that right, you practically guarantee them all surviving the final mission. The only thing left is to choose the right character for the right job.
Tali and Legion have strong tech hacking skills.
Garrus, Jacob and Miranda have strong leadership skills.
Samara is the best biotic shield option. Miranda is a good second option.
Grunt is the best crew escort option. Mordin is a good second option.
Thane and Jack are best for combat, thus helping you get through the enemies as fast as possible to save the rest of the crew.
Note: If you have anyone who you don't have a strong loyalty or relationship with, keep them as safe as possible. This at least ensures their best chance of survival in the last mission. It sadly won't guarantee them coming out of the last mission alive.
This is the best advice I can give. Hope it helps.
