I have been thinking of going into business for myself. I have been repairing fitness equipment for a living for the past 14 yrs. My questions is, Should I receive payment upfront or is this poor business practice. It's been my experience that the side work I have done in the past, often when people find out how much is a repair for parts and labor they often decide to back off the repair. Afterwards they want to haggle over the money for me just showing up. Since the repair is not completed they feel they should pay me what they think is fair. I always stick to my guns but it's often a battle. I even stopped doing sidework for this reason because I didn't want someone go to the company I work for to complain.

Make them sign a contract before you do anything. Pay a lawyer to draft one up.

X amount to diagnose what all it needs payable in advance... credited towards cost of total repairs if they agree to proceed with repairs.

Take them to court and use it to sue them if they refuse. Even to file a mechanics lien against the business or them.

I see no need for a lawyer for a simple contract format.
Your dilemma is the same for most people working on their own. There is no easy solution.
I would get the diagnosis cost paid in advance, before you even go look. You can write a clause for very simple fixes, in which part of the diagnosis cost is applied to the final cost.
Have a column for parts costs, just like auto repair shops do.
Sit down and make a list of all the troubles you have had with customers, and incorporate them into a good repair form.
Give a 10% discount for payment at completion of job.
Give 5% discount for payment within 10 business days.
Charge 1 1/2% per month for any payment later than 30 days.

The reason I suggested having a lawyer look at it... is to be certain there are no legally unenforceable clauses in it that might have the court nullify it. Might never happen, but for the fee, the peace of mind could be worth it.

Except it's not such much a contract as a simple work order, and the main problem is getting paid for looking at the job.

I forgot to mention that if your employer finds out about side jobs, he might fire you. I guess you know that. But it doesn't always take a complaint for word to get out.

If they want to try and discuss the price, then part of it, is your own fault, you just say, this is my charges. Make sure there is no misunderstanding on your service fee, (cost to show up) and so on.