You need to find the best tradesman that you can, they need to be reliable, professional, clean and tidy. Before you have a maintenance issue invite a few local tradespeople to your property, tell them you are looking to build relationships with local tradesman who can assist you when matters arise. If you do it this way you will have a list of tradesman you have already met who you can call in an emergency. They will have already met you, know that you are a professional landlord and will want to impress you.
You could join forces with other local professional landlords and invite tradesman to be on all of your lists.
Ask to see examples of their work, particularly if you are looking at larger refurbishment projects. Ask for a price list so you can have an idea of what you should be paying for certain items.
Create a note in your diary for seasonal items that you should undertake to minimise maintenance issues that may result. After the Autumn it is advisable to have the gutters cleared, before the Winter it would be a good time to get the boiler serviced. In summer you may need to remind your tenant to keep an eye on their garden or if it vacant tidy the garden at this time. If you are proactive in your care for your property you will save money in the long run.