There are two things necessary for digital reception for ATSC.
One is actual raw signal level. While not nearly as important as it was for analog, you need to be 15 dB above the noise floor (20 dB works best in practice as that gives enough headroom to cover momentary dips in signal level caused by atmospheric and other phenomena). You also need to be below the level where the front end of your tuner overloads; some reception problems are fixed by actually lowering the signal level with an attenuator. So there is a window of operation. Not too much, not too little, just right, like Goldilocks and her bear friends recommend.
The other necessary thing is a high ratio of freedom from multipath interference. The more directional your antenna is the more it can reject interference from other directions. But the more directional it is, the higher the output signal level it generates is as well, so you might need to have a directional antenna as well as an attenuator if you are within the 10-mile or so range. Newer tuners can tolerate a lot more interference than those from just a few years ago, which is why line of sight, while helpful, is not really necessary anymore. Not having it does increase the amount of interference (the ratio of interference to signal).
So other than aiming, and using a VHF/UHF antenna, there are other things you can do. Try to use the most directional antenna you can get. Add a FM trap, as there are a lot of high-power FM carriers coming from S. Mt, and this will help lower the noise floor and improve reception on marginal channels. If you use an attic or set-top antenna, realize that metalized window film, metalized insulation, and stucco gridwork acts like a Faraday cage (Google that) and lowers the amount of signal you get. Also, each frequency might have a different interference node at a different physical location, so that explains why you get one channel with the antenna in one position and a different channel when in another. There is probably a location that can get both; you just have to find it.
Building the best antenna system and downlead/distribution you can should be done first, and this is in the interests of getting the highest signal with the least interference. Then you can move to the peaking technique below, which is done to get the signal level to whatever the most effective level is at the input to your amp (if you are using one) and at the input to your tuner.
Once you get set up the best you can, here is how I like to aim and adjust for best reception: Put a variable attenuator in the line just before your TV, turned to minimum attenuation ($8 at RatShack). Tune to your hardest-to-get channel, and peak your antenna for the highest reading on your digital quality meter (most TV's have a screen for this built in). Note that this is different from a signal LEVEL meter (which some TVs also have).
Now start slowly adding attenuation. If the digital quality level only increases, you know you have too much signal and will need a permanent attenuator. If it only goes down, add a pre-amp (typically only for distant reception) or a post-amp (typically only for a signal that is split to multiple TVs) and try again. If you use a post-amp or line amp, experiment with the attenuator on both the input side and then the output side of the amp.
If the attenuator shows its best reading at full attenuation, you either need more attenuation or you might need to remove or disable any pre-amp or post-amp in the signal chain. If the attenuator shows its best reading at minimum attenuation, remove the attenuator and take it back to RatShack; you don't need it, but you may need an amp, so don't take it back until you are sure you don't need it along with the amp. But if the signal first goes up and then starts to go down as attenuation increases, you know you are in the zone, and you can simply peak for the highest reading and leave the attenuator in line.