Try to teleport to an unexpected position before trying to transform.
And people would be surprised how much less noticable they are if they don't swoop or turbo; models were shrunk from average Half-life models on 1.1 I think it was. The end result is that at far distances, it's quite hard to see someone.
I've evaded people completely by just turning flight off.
No sound, no swoop aura, and it baffels them completely.
Letting the other person transform is a matter of respect.
In the cartoon, there's a degree of respect between (most of) the fighters.
At least, that's the premise, anyway.
Don't expect a 14 year old from Mississippi to have the same respect.
You have to
A) Hide
B) Go across the map
or C) Get more PL, so transformation takes less time.
With trunks it shouldn't be hard to double your PL again, and have instant transformation.