Every agent should have a team that surrounds them. I don't mean the team that you are probably thinking. You don't need to be doing 50 deals a year with an administrator and junior buying agents working for you, in fact, whether you have been working for 1 month or 10 years, a team is crucial to your success. This team is the professionals you rely on for their expertise in their fields. You don't know everything and aren't supposed to, which means in order to help your clients, you need people who you can lean on when questions outside of your scope of knowledge comes around. It's good practice to have at least 3 people on your list to recommend to clients, you may only have experience with one or two, but providing at least 3 will keep you out of trouble.
A bonus for this that you become the person people (*hint: potential clients) turn to when they need someone. If you know and can recommend a trustworthy person every time someone asks, eventually they'll just keep asking you... and that's fantastic as a way to maintain a connection!
Here are the minimum people you should have a good relationship with:
Despite not needing them immediately, here are a few people you should have on your team:
You won't necessarily need all these people tomorrow, it may actually take a few years of building relationships, but you should definitely spend the time to meet as many of these professionals as possible so that when the time comes for a recommendation, you have someone to recommend.
Additional note: You may not need them immediately, you may not use them often, but check in regularly. See how each of your contacts are doing, what they've been up to, how busy they are. It's important to maintain a relationship with these individuals because if you are going to recommend one, you should know how their business is going!