My mouth is dry quite often. What can be the reasons for it?

Sjogren syndrome. It is important to assess whether this is medication-related or to an inflammatory disease called sjogren's syndrome. If dryness is pathologic, oral hygiene is critical to minimize tooth loss.
Nasal Obstruction. If your nose is clogged then you breath through your mouth. That can lead to significant mouth dryness. There are also many medications that can lead to dryness as well as certain medical conditions. If this is something that bothers you then you should consider consulting an ENT doctor for an evaluation.