Perhaps i can help quickstep here. It is a difficult topic because it's really all about 'feeling' the music and interpretation.
However, here's some thoughts which i've found useful.
Music is usually written in 8-bar sequences, but it might help to think in 4-bar sequences too for slow waltz. The end of bar 4 is often 'weak' and bar 5 is strong again.
At the start of an 8-bar sequence it's best if the man is going forward, because the music is usually making another 'statement' (starting a new melody line, or repeating a melody line)and a 'strong' movement is needed. this usually involves the man going forward, especially for turning figures. So for example, a natural turn would be a good start for a routine, and many top dancers do this.But if you follow the natural turn (2 bars)with a link step(1 bar) you're on this weak bar 4 for the start of the reverse turn. So this wouldn't be the best choreography in the world, but it would still be good, and only a championship grade dancer would be too concerned about it.
The 8th bar is the end of the sequence and is usually weak, like the music is drawing a breath before continuing. so think how you would do this in your dancing. Which are the 'drawing breath' figures? Can i suggest opening into promenade position is one. So, as quickstep mentioned, you could go into a line step, such as a throwaway oversway, and recover to come out in promenade on bar 8. Then come out strongly, say with a passing natural movement into a corner. This is what the top waltz couple did at their lecture at the Blackpool congress last year (2006)following it with anothe line figure for 2 more bars, to get back on a strong bar again to come out of the line.
Probably any part of a figure where there is little travel would be good as the end of an 8-bar sequence.
So, it's up to you to decide 'strong' and 'weak' figures or parts of figures and match them to the music.
Now the really hard part. If you're dancing a pre-planned routine, you can of course phrase it out in advance. But what if a couple blocks you in a comp? now you've got to adapt and change your routine to match the music. You will need to develop the skill to 'know' the end of the 8-bar sequence is coming, and dance accordingly, maybe staying in that throwaway for an extra bar (and looking like you always did it that way, of course!).
Good phrasing is even more rewarding in foxtrot of course.
If anyone has more, and better, ideas, i'd be very interested in hearing them. Interpreting the music is really what dancing is all about. When it goes right, it's magic.