The people, who are living in rented units are obligated at the end of the contract to perform thorough cleaning. This is part of the contract and they either have to do it by themselves, or they have to look for help from professionals. In most cases the contract obligates them to do it the second way. This will guarantee the landlord that at the end of tenancy clean and nice apartment will await the next tenants.
Of course it doesn't mean that you have to just sit and do nothing during your time of residence. Even if you have to purchase the cleaning service it doesn't mean that you can't do something to keep the place as clean as possible.
One easy thing, that you could watch over are the windows. Here is some advices, that you could put into practice. Before you start any treatment you could dust them off. Simply because if there is dust when you start with the water it will get pretty muddy. When you are considering buying squeegee go with the one for 20 pounds. The dogma the cheaper, the better doesn't work here. Pick a cloudy day to start cleaning the windows. The direct sunlight will dry them off really quickly. Use a cotton cloth to wipe off the water residues. Line the ledge to make sure there is no filth have left.
By doing this procedure at least once a month there shouldn't be an issue when it comes to the final cleaning. You will not have to put any special cares cleaning the windows. And if the professional end of tenancy cleaning service allows exclusions from the package you will be able to safe some money from that too.