We've made a few trips and on the last one we asked one of the cruise director's staff (whom we'd met before and got to know fairly well) 'How come we always have such a wonderful time?'.
He answered that it was because if the crew like you, you will be so well treated that it WILL make a difference.
That's not to say that the crew will not look after you otherwise, it's just that they DO notice who appreciates all that they do.
For example, one day we were having a drink at the bar and got chatting with the Food and Beverages Manager. He asked us how we were finding our trip and was everything ok etc etc. We told him what we thought of the trip and said it was like being in heaven.
That evening a bottle of wine was waiting on our table from him with a nice letter of thanks for our appreciation.
On our last trip, the last night was drawing to a close and I asked the barman for the bill.
He told me that the crew were paying that one.
Tips probaly do have something to do with it, but of equal (crew can argue whether or not) importance is just your everyday interactions with the crew. Do you wait happily if they are busy? Do you always order with a smile on your face? Do you empathise with them? Do you ask for ten towels a day or do your best to help make their job a little better? Do you leave your cabin 'housekeeper tidy'?. Do you do your best not to make a mess at the dinner table? If something is not right, do you make comment pleasantly?
It boils down to consideration for others. Crew are human too, and whilst you might be paying for your cruise, it pays dividends to pay attention to the crew and the excellent work they do.