(In my humble opinion) the current level of seasonal staff would not be necessary if there was a greater level of productivity within all rosters. The 'powers' that determine how much work cabin crew undertake are never likely to agree to a increase to a two-sector day - unless more 'obstructions' can be implemented as a 'concession'.
Dave, contrary to popular belief you have the most control over your fate. Your best course of action is simply to show a greater level of enthusiasm than your counterparts! Most of the time it only has to be relative to the nature of job you are doing! The rest of it pertains to common sense; such as limiting 'sickness' to days when you truly are sick! Earning the odd ‘good report' from one of many sources will do your cause no harm either. It's also worth having a word with a few of the faces that were kept on permanently from previous summers and seeing what worked for them.
Good Luck