How to plan for early retirement?

I want to retire in 20 years at age 55. I contribute 25% of my salary to company 401(k). What strategy should I employ to save enough to cover the 4 1/2 years so I do not have to withdraw early from IRAs. Reduce contr. to 401 and invest in taxable mutual funds? ume I do not have money available above and beyond the 25% of salary.