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Hi Andrew, I'll jump in before someone else does.
Seeing conditions will vary greatly from session to session. When the seeing is poor, you may be limited to only using your low power eyepiece(s). In very steady conditions you may get to use high power eyepieces.
When there is a large diurnal temperature range (summer), seeing issues will be worse, especially in the early evening. It is exaggerated in the early evening when you look over surfaces such as roofs where a lot of heat has been absorbed during the day. You will find seeing is much steadier in the wee hours of the morning.
Winter is generally steadier than summer as far as seeing is concerned.
Another issue is the thermal currents created in the tube of your scope when the mirror is more than about 2.5 degrees warmer than the ambient temperature. Generally in the early evening, the mirror will not keep up with the rapidly dropping external temperature, and so you will get poor seeing within your scope.
Sometimes the seeing is so crap, I give up. Other times I leave the scope set up, go inside for an hour, and come back to see if things have improved. You take the good with the bad.
Another trick is to get your scope outside as soon as the sun goes down, so that the mirror has the chance to start cooling as soon as possible.
Hope that helps!
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