Hydration levels are crucial. If you are dehydrated by as little as 1% this can result in 'up to' 10% drop in concentration and performance. The brain is made up of so much water that even a little drop in hydration levels will result in slower electrical impulses which in turn can give slightly slower reactions, you will feel fatigue earlier, your timing is off and you generally can feel lethargic. All this from not drinking enough water.
Players should drink before a match and between games and that doesn't mean cordials either (as this is a diuretic - i.e. makes you go to the loo more and hence dehydrates you further). Also most teas & coffees contain caffeine which also dehydrates you.
Allied with this is energy levels through your dietary intake - get your energy foods wrong and eat rubbish sugary, fatty un-nutritional foods all the time and you won't have the energy to concentrate and sustain your standard throughout any long game.
Another area is sleep patterns and the amount of sleep you get. If you know you operate well on 8-9 hours, if you only get 6-7 then this can dramatically affect your physical & technical performance.
Avoid running for moderate to long distances, this isn't very common amongst the top players. The squash court seems to be the main focus for most, as regards conditioning work e.g. ghosting, court sprints, pressure sessions
Improvement often comes in fits and starts. Often there is a levelling out, after which
significant improvement may be made. Consistency in squash and in sport, maybe in life
generally, is a virtue and the best way to achieve it is by regular hard work, if possible, well directed. Of course being erratic is often a question of temperament, though whether that is the cause of the problem is up to the individual to decide.
There is no particular level that a player should begin to solo practice but obviously the standard of the player will determine what practices they may be able to cope with.
If one likens it to soccer lots of small kids will familiarise themselves with the ball with endless hours of 'keepy uppy'. The same can apply to all sports and at the beginner level it is very important for the players to have many contacts on the ball as possible. This maybe difficult when playing a basic game as the players may not be able to keep even short rallies going so some solo practice on court would be very beneficial
With regard to stamina increases it can be dealt with in sections, but it depends on what part of the season you are presently at. If you are preseason then an endurance running program will not affect your match results, however if you are midseason