Be sure you salt the water when you poach the chicken. That's VERY important. It doesn't have to be overly salty, but like you would salt it for cooking pasta. First bring the large pot to boil, then add salt, then add the chicken breasts (boneless and trimmed of fat) into the water and put the lid on for 25-30 minutes. If you use tenders, cut the time to 15 minutes.
Remove the breasts from the water and let them rest and cool enough so the juices don't run when you cut them up and they can't be handled without burning your fingers.
If you're going to cut them up or shred, add the sauce or seasoning or marinade after cutting and marinate.
You can marinate whole pieces that aren't cut, but I'd start when they're still warm and do it overnight. Then just heat them up in a skillet WITH the stuff you marinated with or put the pieces on the grill, slathered, to get the grill marks.