The factors influencing credit are quite incongruent in some cases. Too much unused credit? Bad. Not enough credit history? (Bad, probably the worst characteristic next to prior delinquency/bankruptcy). Someone who has always been a cash buyer of everything, and is self employed, is viewed as a high credit risk. there's just no (or little) sample size to evaluate.
There's a fundamental sweet spot when it comes to use of revolving credit. The ratios are what you need to pay attention to (if you're trying to improve your FICO). If you utilize 75% or more of available revolving credit, that can ding your FICO. Bring that utilization down if you can. If you're north of 750, you have 'great credit'. There are no brownie points for being 800+ (850 is generally the top), though a couple 'newer' scales go into the 900s. Truthfully is you are 720 or better, you're going to be fine.
Pay on time (always)
Establish and use some revolving credit (hopefully you pay off each month), keep utilization close to 50%
Installment credit (student loan, mortgage, HELOC, with consistent payment history) are usually positive credit score factors.
Order your report once a year (check for accuracy, mistakes, fraud).