Use the following checklist - it works every time in my experience.
1. On first arriving at the aircraft, before the cc have had the chance to say 'there is no water pressure so you're not getting a drink', tell them you are switching on the APU just to keep them warm/cool (delete as appropriate).
2. Smile at them.
3. When they ask you if you'd like a drink, say 'tea, 2sugarsplease'. (none of this coffee crap).
Now here comes the important bit.
4. Be polite at all times.
The result is an excellent cup of tea every time from someone who does a far better job with a teabag than I do. (I don't have the patience to dunk it long enough).