When I am on the dolmuş, I never know what to say to the driver when I want to get off the bus.  

What would be the appropriate phrase/phrases to use?

You can say; Müsait bir yerde inebilir miyim? or instead of müsait maybe it could be easy to use "uygun" for you. Like this : uygun bir yerde inabilir miyim.

Turkish people generally say: " Müsait bir yerde inebilir miyim!" or "Müsait bir yerde inecek var!" or "Durakta inebilir miyim!" or "Durakta inecek var!"

These sentences are correct.

If you want to get off in a specific point, then you can just say " Burada  inebilir  miyim ?" [ Can I get off here ? ]

Markette  inebilir miyim ?  [ Can I get off  at the supermarket ?]

or 

Markette  durur  musunuz ? [ Could you stop at the supermarket ?]

sometimes we are being so lazy that we just say ;

Markette  [lütfen] --> At the supermarket please.

Most of the time, we end up getting off  at a bit further place than we wanted.

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note : Sorry guys, i couldnt hold myself , forum is so attractive and addictive  

Yaşasın,yaşasın!! Tunci geri geldiiiiii...

Just a useful tip in Turkish dolmuş ;  If  Dolmuş is packed with people and you are sitting in the back seat  then it is better to get up earlier before  you reach your destination. So that you will have better chance to approach to the front door and it will cause less chaos in your getting off process.

Another phrase we use when we hand the money over to dolmuş driver ;

* If you are handing your money directly to the driver ;

 1  kişi  lütfen ---> One person please. 

 * If you are sitting far from driver, you get people sitting in front of you to pass the fee for you saying ;

 1 kişi uzatır mısınız ?  ---> Would you pass fee for 1 person ?

 1 kişi uzatabilir misiniz ? ----> Could you pass fee for 1 person ?

Dolmuş drivers are sometimes don´t hear what passengers say, so  try to be bit loud when you ask him to stop.

And try to give him change money as it will save you from the process of waiting your change back. since sometimes the driver may confuse with giving people the right change back.