Don't stray from the original requirements to do work that is sexier, or is the result of scope creep. I've seen this time and time again. Especially with those new to freelancing. This is not a problem limited to freelancers though. Complete the initial task first. Before you run out of time and money. Then go back and add the polish. At least when you finish, you'll have a usable product ripe for an up-sell.

Sometimes it's hard to stay on track. Especially when your client keeps asking for work to be done outside of the scope of the original agreement. I get it. It's hard to say no at first. Get used to it though. You need to think about who he is going to blame when the project is late. You'll know why it was late, and you can tell him. It won't matter. It can still lead to you getting fired from that project. Or if you're an employee, you won't win any brownie points. Sometimes you have to manage your boss. When I'm asked to do work outside of the original scope, I always make it clear that we can address it once we get the first priority out of the way. If I'm working on a fixed price bid, my response is, "I can get you a quote for that". Usually that makes the priorities clear. Even if I'm working hourly, I make it clear it will not be started until the task at hand is done. Sometimes there is a real need to interrupt the current task/project and start something new. Putting up road thinking blocks helps the client understand his actions better. It means synapses need to fire before making the decision. He can better associate the decision to stop the presses with being his own.