1) You have to invest your savings in something that will at least keep up with inflation. A 5% CD would do.
2) You need to increase your monthly savings at the rate of inflation. So if you start off saving $1000/month and inflation is at 4%, you should be saving $1040/month a year later. Hopefully, your job will give you pay raises that will outpace inflation.
If you do these two things, you should be able to accumulate your %20 downpayment in less than 8.3 years.
