When I sold satellite dish systems for a living the first question I always asked a prospective customer was if they had clear skies to the south ( for DirecTV ) or southwest ( for Dish Network ). Actually, southwest for Dish Network applies if you are east of the Rocky Mountains and changes to southeast as you move west of the Rockies.
There are usually two culprits when it comes to line of site issues, trees and apartment or condominium units with balconies facing north. If you live in an apartment building that has a balconey facing North, it is highly unlikely you can obtain a satellite signal. The only way to do it is to mount the satellite dish on a pole and try to shoot over the roofline ( ocassionally works if you are on the top floor ). Just make sure you mount the pole very securely so the satellite dish doesn't turn in a strong wind.
Trees are another matter. If you live in an older neighborhood with large, heavy trees overhead it can be very difficult to obtain a satellite signal. DirecTV comes down at a steeper angle than Dish Network (their satellites have a higher orbit) so it is the best choice if you have heavy trees to the southwest ( southeast if you are west of the Rockies).
Sometimes you can go to the very peak of your house to clear the trees, but this is considered a non-standard installation by most installers and you may be charged extra. Some installers will not do a non-standard install so it's best to ask prior to completing the purchase order. Most retailers will try to accomodate you if you make it an issue at the time of the sales negotiation.