Manually focusing tips?

So, I'm currently using a Nikon D3000 and have begun shooting manually since I have gotten used to the aperture and shutter speed and whatnot.

However, while I do know how to take a nice picture, manually focusing is what is bringing me down. Since the Nikon D3000 does not have the ability to shoot video, I'm forced to use the viewfinder to focus my photos.

Sometimes it becomes too difficult to see through the viewfinder and I don't exactly know if the photo is focus at that moment unless I take it and make sure.

Are there any tips on manually focusing aside from trial and error?

The best thing is to get a viewfinder "cup" that fits over the eyepiece and shields it from extranious light ingress.

Alternatively set the zoom lens to the level of "zoom" that makes the subject appear clear in the viewfinder without surplus background in shot then adjust the focus so that the subject is sharp. Then zoom back out to the desired frame and shoot.

This works with most decent lenses that maintain focus throughout the zoom range but it may not work well with cheaper lenses.