Ah, I remember those days and bloody infuriating they were too! The only bone I can throw is that the more you do it, the better you get at it - sorry, it really is a case of practise, practise, practise. What I will say, which is something no one told me way back when, is that the better you get at it, the less you tend to play all six strings anyway. You learn to use the flavour of the voicing from just four notes (sometimes less) and rarely invoke the rowdiness of an all string voicing (Andy Summers is a perfect example of this in songs like Message in a Bottle).
In short, keep at it, the strength will come as with most physical activities.