Notify border control on additional visits

Do I have to declare the intention (e.g. tourism) for additional visits to border control? If yes, how should I do that properly?

You may be asked to declare your intention. To do it properly, you should respond to the immigration officer, who will ask "what is the purpose of your visit" or something to that effect.

So having tickets/hotel and similar proofs is enough?

It should be. In general, you should have the same sort of evidence supporting the premise of your trip as you would present for a visa application. The usual advice is that you should present this evidence if you are asked for it rather than volunteering it up front.

The general principle is that you should avoid volunteering information, other than presenting your passport when you step up to the desk, but you should not hide anything if asked. Above all, never lie. Immigration officers do not like being lied to.