There are 3 main objectives with tax planning: 1) increase your deductions 2) capitalize on tax credits and 3) decrease your income.
Regarding the first point, there are myriad deductions people don't take advantage of because it can become complicated if you've don't know what you should be tracking. Depending on your line of work, deductions may include assets to work from home, commuting, personal property taxes, investments, or even repairs & maintenance. The best way to identify these deductions is working with a CPA as an automated software may not recognize your situation.
Secondly, there are numerous tax credits people don't take advantage of simply because they don't know about them. For instance, there are tax credits for taking courses, college loans, or caregiving.
You can reduce your income with a 401k or IRA. This makes your total taxable income lower which can be paramount in reducing your tax bracket. If you're in the middle of the tax bracket, a 401k may be more of a challenge, however if you're just shy of qualifying for a lower tax bracket, a 401k is a must. This method can reduce your AGI (adjusted gross income), thereby reducing your taxes.