If you're interested, I could explain exactly why those sticker aerials may make things better or worse. In short, if the sticker has parasitic elements of exactly the correct length of 1/4 of the wavelength of the frequency you're transmitting/receiving on, then it MAY act in a similar way to a Yagi array assuming the sticker is EXACTLY the right distance away from your phone's antenna, and the phone is oriented in such a way that the phone's antenna and the sticker antenna align perfectly to point directly towards the cellphone mast you're currently using.
In practice, this simply doesn't happen, as every mobile phone model has its antenna in a different location, and without taking your phone apart most people wouldn't know where it's located. Even then, you'd have to by brilliant at geometry or use a dedicated computer program to figure out exactly where to stick the sticker, and after that you'd have to know the exact location of the cell phone mast you're using to line everything up. If things aren't positioned in exactly the right locations and orientation, the signal will be the same, or even worse.
There are some other options though:
You can buy devices that act as an passive repeater, either powered (where it received the mobile signal via an external antenna on the roof of your house or wherever, amplifies it, and re-transmits it into your house/car), or you can get truly passive ones that basically use a big antenna to amplify the signal where it can get a strong signal (roof), and direct it out of another antenna near to your phone (a room in your house or your car). The former of these are expensive (hundred to few hundred dollars), the latter you may find for $50, usually as part of a car kit, but they're crap.
You can get what's called a GSM picocell. These connect to a phone line or broadband connection and act like your own mobile cellphone mast (only with a range about the same size as your house). These cost hundreds to thousands of dollars, and often present themselves as a different mobile network, so you may need an unlocked phone and a special SIM. I understand some just "repeat" the standard mobile networks, but I have no experience with them.
BlackBerry devices are known to use WiFi for stuff. They essentially have servers on the internet to handle SMS and Voice data, so they use VoIP and internet SMS gateways when there's no normal signal. You can often send SMS messages from various websites (T-Mobile UK can do this, but the reply still comes to your phone). MSN, Yahoo, and other companies have SMS gateways, some free. You can use Skype (I think?) on Android devices, and there are other VoIP services like Google Voice - I believe these all offer SMS and Voice using data (someone correct me? I've never used them on an Android phone).