In spite of what their large size may suggest, pianos can be delicate instruments. If your piano has spent some time in a storage unit, you don't want to just pull it out and start playing it immediately. Being exposed to fluctuations in temperature and humidity during storage can cause some stress on the piano, so you'll want to take these three steps to minimize the effects of that stress before you start playing.
Vacuum off the keys and any other vulnerable areas.
A little dust on the surface is not a big deal, but if you start playing the piano when there's dust on the keys, that dust may works its way down between the keys where it can cause friction and interfere with the way the piano plays. So, vacuum off the keyboard using the wand attachment of your vacuum before you play. You should also vacuum up dust that has accumulated along the back of the piano where it opens so that when your tuner arrives to open the piano, this dust does not end up flying inside.
Have the piano tuned.
Changes in temperature, humidity, and pressure can cause the strings in the piano to stretch or shorten, which causes it to go out of tune. Hire a professional piano tuner to come adjust the strings and ensure the piano plays true. Piano tuning generally costs around $100 per hour, and a thorough tuning should take an hour to an hour and a half. However, this is money well spent since nobody wants to listen to an off-key piano, and playing one may cause you to lose your "ear" for the notes. Tuning also protects the piano's inner workings from premature wear, extending its life.
Polish the wood.
Once your piano is tuned, it's mechanically ready to play. But you need to ensure the wood is kept in good shape, too. Purchase a good quality furniture polish, and apply a liberal coat to all of the wooden parts. If you see any little cracks or dry spots, focus on those areas in particular. You may want to repeat the polishing process a week or two later if the piano has been in storage for a long time and the wood has become noticeably dry.
If you take the time to properly "rehab" your piano when it comes out of storage, it should keep on serving you for many years.