I know that there has been a method given on the Microsoft Windows Phone website about how to install apps in XAP formats from SD Card to your phone. Just keep these things in mind:
1) NO, you will NOT get the full version of paid apps via XAP, you will have to download it frmo the store on the phone. Even if you find XAPs on other website claiming it to be full version, it will not work.
2) If your XAP apps are not being displayed on the store, or the SD Card option is missing on the apps, restart your phone, wait for a while(2-5 minutes) and then start Marketplace app. This problem occurs in mid end phones like Nokia Lumia 620, or lower end phones.
To install apps from SD Card, follow the tutorial given on windows phone store here.
Why this method:
1) The Windows Phone Store downloads are very unstable on phones, so downloading them on PC is much more faster and less head ache giving method.
2) Saves data costs, duh! Everybody loves saving money, so do it this way too.