OP. Put the bike in a high gear, one or two down from your highest gear, then lean forward and slowly lift your rear out of the saddle.
Too low a gear and you risk a dangerous wobble as you try to stand. Too high and you risk turning the pedals too slowly and losing balance.
Are you changing position, with hands on the drops, on the hoods, on top of the bars?
You probably don't need to stand; maybe, though, you should get off of the saddle by stopping for a minute or two at the 20 min. mark, standing over the top tube or even getting off the bike. Over time you - or your fanny - will get used to spending more time in the saddle on your trainer.
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