This is a trick question :-)
Let's set it up as a do-until loop:
seek to improve
until improvement noticed
But that's not what you want to say, right? You want to say "always seek to improve" or "never stop trying to improve". Or do I misunderstand you?
So, your "still" is important, and your choice of tense is important, and the word "until" cannot be used.
The perfect won't do here, because, as its name suggests, once perfected, the person would have to stop seeking to improve.
The word "until" wont' do here, because it places a limit on the action. We must use "always" instead.
An idiomatic way to encourage people not to give up, and to seek always to improve is to use the phrasal verb "keep at".
Keep at whatever you do and always seek to achieve still better results.
Keep at whatever you do and always seek to achieve even better results.