My method is this- I try to keep the steps the same and in the same order for each patient, that way I always know what I'm going to do next and I don't leave anything out. I can also ask all the quick short-answer questions as soon as the patient walks in and write their answers down as they speak. By getting all those out of the way before asking the patient relatively open-ended questions about why they're here, I can keep things focused and also be typing everything else up if they turn out to be an incurable rambler.
So it goes like this, usually:
"Hi, have a seat, I'm emmy27 and I'll be triaging you today. Full name and date of birth please? Are you allergic to anything? Oh, what happens when you take that? How tall are you? How much do you weigh? Do you smoke, drink, or use drugs? When was the first day of your last menstrual period? Do you have any medical history? How about surgeries?"
This all takes (usually) under a minute to get through, and while I'm doing it I'm also putting the bp cuff and pulse ox on while they're telling me their name and dob, and getting the thermometer ready. I write down their answers on their sign-in sheet as I go, and take their temp last (so it doesn't interfere with their answering), and write that down as soon as I get it. So within one minute of them walking in, I can sit down in front of the computer with 90% of their answers already in front of me. This has proven helpful many times when something went wrong with the computer and I had to finish entering the triage later, or when somebody with a stroke/MI/GSW walks in in the middle of a triage and I wind up distracted with that before I'm able to enter the triage in the computer.
*Then* I ask the patient what their primary complaint is, and if they're not giving a concise answer I use specific questions and redirects to get them to focus ("When did that start? How would you describe the pain, sharp or dull?" "Have you had n/v/d with that abd pain?"). Patients don't know what information is pertinent and so they'll often over or undershoot (either "I just feel sick." or "Well it all started back in 1993...")
As far as second-guessing where you're putting them, it helps to think through what the likely resources needed will be (just like in ESI training- somebody who likely needs every treatment and diagnostic in the department is gonna be a higher priority) and what the worst case scenario is, and whether there are warning signs (history, vitals out of range, just a bad gut feeling when you look at them), and how they compare to the other people you know are waiting. It's good to glance at who's still in your waiting room between patients so you don't forget about them. Usually it's pretty clear if you just consider who you've seen today which among them is your top priority for going back next, which with limited resources is your real question. They all have to go back eventually, you just have to figure out who needs to be seen *first*. It's okay to change that order if you notice changes in the patients, too, and if you're really on the fence about somebody, you can call them back in to recheck their vitals and assess their pain later- you don't have to make a call and then live with it for eight hours if they're deteriorating. And if they're not deteriorating, congrats, you made the right call.
If you haven't had ESI training, talk to your educator about it- they have an excellent online module that's really, really helpful in learning to think systematically about triage.