Hi, I'm Simskin and I'm a WoW addict!! Knowing there is a problem is the first step to solving it :-)
These guys posting are some of the best(helpers!). Teacake has got it down, always so clear cut. Mozared is great too, as all the posters here.
My advice? Change your UI. Yeah, it sucks doing so and adjusting is a pain in the bum, but eh.
I personally use Omen and Healbot. Omen, well the new one doesn't have a healing mode (unless I'm using it wrong) so you don't see threat unless you are targeted on the MT's Target, BUT it does make a sound and flash and even print on the screen when you pull agro. Yeah, you see that and then lose focus. I understand that
all too well. But it is a great start. When you see it, take 3 sec and prepare yourself. First Shield, then Fade, maybe pop a renew on yourself, then get back to your job (see Teacake's post for more info there). I do understand that the sign of a good healer is to keep your group alive, but if you are not alive to do that you've failed. Most of the time when you are facing this problem.... Yep, tanks forget or get busy and lose track of those casters. That (as Mozard said) is the tanks fault. It's their job to keep the mobs off us and you need to vocalize it to the tank before/during/after it happens to help solve that. I try in a teacher/caring way to let them know in a whisper (so as to save face in front of the group).
I understand that X-Pearl and Pit-Bull and stuff like that are great and all... but too much info is just as bad as not enought. There is a balance I guess for each player. I started out using X-Pearl, Omen, and Healbot. I found that, personally my screen was just too messy and making things small just made me squint more. I dropped X-Pearl and closed the Omen window (toggeling it when I spend more time slaming target with Smite and such). Even with the window closed it will still let off those wonderful warnings. I've actually removed the Blizz party screen. Yep. Gone. Great thing with Healbot. I did at one point remove mine too, but I liked the other functionality of it (easy mana read, in combat, ect... it also gave me 2 places to see myself).
I think it's a Blizz UI, or maybe Healbot... but you can set your screen to do a VERY ANNOING red flash when you reach a certan health %. Or maybe it's a combo with Blizz UI (the flash) and Healbot (the % it starts going off)... either way, set it higher. I have mine set around 35% so that my screen turns red reminding me that I'm dying and need to do something about it.
Another great thing about Healbot, my toon is on the top of the list (the second place I can see my health).
Here and
here are screenshots.
If you can't tell, I recomend Healbot as your main UI and that you use Omen for help with threat.
GL and happy hunting.