Even if you have a 'day job' of sorts, it's always imperative to remember that landlording is a job, not a hobby. You may not use it as your primary source of income, but that moment that you buy a property with the intention to rent it out to a tenant, you've decided to take on a second job. Hiring a property manager or a property management company will help add a layer between you and your tenants that you may or may not want, but if you choose to work with them directly there is a certain level of professionalism that will be required. While three-piece-suits are not necessary, showing up in a t-shirt, shorts, and flip flops to a lease signing or to meet an applicant who is providing you with their application is likely not appropriate. You don't need a corner office and an assistant to create a respectful landlord-tenant relationship. In fact, many landlords that only own a few properties will work from a home office, but you will still need to keep the boundaries solid. Professional attire and a professional attitude will go a long way. If you lack these, they may still decide to rent from you, but your new tenant may subconsciously see the relationship as something more lax than the business relationship that it should be and that could cause you trouble down the line.
While you should always be polite and friendly with your tenants, that doesn't mean that you'll become their best friend. Keep that relationship professional so that when they come to you with this excuse and that excuse as to why they can't pay rent on time and you should give them 'just a few more days' you'll know when to put your foot down and be firm. If the tenant believes they can take advantage of you, many will, and a professional demeanor will help secure this idea in the tenant's mind from the first day onward.