The secret your cellphone company doesn't want you to know
If you're an iPhone user, you need to go into Settings, and then tap Privacy. Scroll all the way down to Advertising.
You'll see a button that says, "Limit ad tracking." If it's not showing a green color, slide the button so that it shows green. This will stop ad companies from tracking what you do with your phone and serving up targeted ads.
Right underneath that setting, you'll see the "Reset Advertising Identifier" option. Tapping on that will zero out the anonymized identifier linked to your personal data on Apple's servers.
In other words, to trackers you'll appear to be a new user. This can make it more difficult (but not impossible) for advertisers to build up a profile on how you browse.
To turn off the Google "AdID" system, you don't go to your Android phone settings, but your Google Settings app. You might have to look under your full list of apps to find it.
Once you're in Google Settings, tap the Ads link and then tap "Opt out of interest-based ads." You can also see your advertising ID and tap "Reset advertising ID" to make a new one. This will make you look like a new user to advertisers.