Another common mistake made by buy-to-let private landlords is:
3. Bad choice of letting agents.
Letting agencies offer a range of services from full-blown management, (especially useful for landlords who live some distance from their property to rent) collection of rent, tenant finding, accompanied viewings, tenant reference and credit checks. They can save you a great deal of time, have a ready list of contractors to carry out maintenance and are usually full of very useful local knowledge which is invaluable, especially if you are unfamiliar with an area.
Choose a specialist letting agent who only makes money from letting and managing properties to rent like yours. An estate agent who also does lettings will put a house buyer before the needs of your tenant. They are only taking small change with lettings compared to property sales from which they get a large percentage of the selling price. Also, currently, anyone can set themselves up as a letting agent with no qualifications or knowledge on the industry and many go out of business leaving landlords in the lurch.
So, do some research beforehand. Go to a specialist letting agency (like Discount Lettings 
), not an estate agent who does lettings as well.
Trust is as pivotal to your relationship with the agent as it is between landlord & agent. Meet several, go with your instincts on who is the expert locally. Be prepared to pay a slightly higher rate. The small difference in fees can evaporate overnight if you are ripped off for tenancy agreements or un-needed repairs!