Do you mean "rarely works at all" or "rarely works well"?
If it's working, but poorly, you could be limited by the upload speed of your home Internet connection, the download speed of your viewing location, contention for the connection or access point by others (common in hotels), interference from other Wi-Fi networks, or insufficient Wi-Fi signal strength.
If it's not working at all: is your port forwarding set up correctly? Problem with frequent IP address changes? Problem with firewall restrictions at viewing location (very unlikely at a friend's, but quite common in public places)?
Please post details about your problem: Upload speed at Slingbox location? Download speed at viewing location? What streaming speed do you actually get? What kind of device are you using for playback? Kind of viewing location (residence, office, public place)? If a wired connection at the viewing location is possible, does that also have trouble? If you have (or can easily install) a Wi-Fi analyzer for your device, what signal strength are you getting (should be -75 dBm or better for good streaming)? Do you see other strong signals on the same channel?