I am now a HLTA, but am also a qualified teacher, i'm just returning to work after a career break. I try to be like the really good TA's I used to have.....I hope I am!
One thing I do like to have is a couple of tray, one for things "in" and the other for things "out". The teacher can plonk anything that needs doing in it, without having to ask, it's usually fiddling lettrring for displays, photocopying, interventions for that day whatever, and then i plop it into the other tray when done. It saves a lot of time, and classrooms can be VERY messy, so it stops things being lost.
I also keep a filing drawer of all my interventions, and a pocket for each child I work in small groups or one on one with. I find it's very important to be otganised, as the teacher will need evidence of what's gone on in interventions for reports and assessments, and it's very helpful to be able to just hand it all over when it's needed, saves them a lot of time.
One things I have noticed is that teachers are very good at sharing resources and things, but TA's seem less so, probably because so many of us are "floating" and going from place to place, so I've found it very helpful to make friends with them all, and offer and share any resources i've made, as i've got some brilliant advice back.
Be prepared enough to do interventions at short notice, to provide off the cuff work when needed, as sometimes it becomes obvious to the teacher during normal lesson time that a particular child/ group hasn't "got it" and so off you will go to help them.
Always make sure you have a whiteboard pen that works. For you and the teacher. Similarly, the amount of time wasted when there are not enough pencils/ pens/ mini whiteboards has to be seen to be believed, check, check, check!
Second the not talking over the teacher, it's really annoying! I like the hover and non-verbal tecnique of stopping off-task behaviour.
I also like to note when it's the teachers' PPA time and see what I can do to help, and similarly, know when "your" teacher is on break duty and bring them a cuppa to the classroom! Lastly, when there are goodies in the staffroom, if your teacher isn't there, bring them a slice of cake so they don't miss out. This is VERY important!