lady is right, the only time i'd ever recommend lying down is in the fake doorway to church if you are american side. but to tell u the truth, i own snipers who do that from the double windows.
from double windows, never walk in or run in, get prone and crawl in or crouch walk in, first thing first is look parallel to broken windows, there is a small crack between your box and the left window to get a clear headshot at anyone in the broken windows. then peak to the right at the orange windows. after, use the sliver between the box and right window to see if there is a sniper trying to get you from allied second. then, now, stand up, this should give you a clear view of entire second, be aiming down already so if there is a sniper in sand bags your crosshairs are already in the right vicinity.
axis - cellar. get prone and sneak in but check orange windows first, then crawl directly left, use the stripe on the allied building to guide your crosshairs to the window. creep slowly. as soon as window is visible stop moving, wait a second to see about counter snipers. if clear unmount scope crawl all the way left. this gives you clear shot of entire second of allies up top.
however, overall, lady and knockers gave the best advice. don't lay down, don't stop moving.
pM me if u want more tips