Drawing the lines
It's a very good idea to draw up the lines softly when you start a drawing - or you might risk getting the proportions wrong. I have ruined drawings in the past by finding out the proportions were off in the middle of a drawing. I used to start by drawing the eyes, shade them and go on to the nose, hair and do the same.
I find it hard to place the eyes, nose and mouth in the right positions if I don't do it to begin with. Unless you have a really good eye for proportions.
It's more important to get a portrait of a family memeber or celebrity right than when you create a fantasy character. It can still be a good idea to check if eyes are in line before starting to shade your drawing. You can find a guide to do so below.

When drawing up the lines only do it softly or else the lines will be too visible or dark and in the end make the picture look unrealistic.

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Proportions - placing the elements of the face correctly
There's 2 ways I know of to do this.
My old design teacher told me a way to place the elements of the face correctly.
Actually it was meant for all kind of objects, mostly outdoor things like nature, but it will work with portraits as well.
You can just use your pencil.

Place your picture/object in front of you, maybe hang it on the wall if it's a big picture. Grab the pencil with you right hand, your arm should be stretch right in front of you. Close one of your eyes(the left) and with the pencil in front of you measure the distance fx. between the persons eyes. It's kind of hard to explain in words, but I hope you understand the point.


Another way - which I normally use - is to simply use your pencil to see if things are in line etc. It helps if you print the photo or whatever you want to draw from the same size as your drawing is going to be. If you use small pictures you can do it too, it's just a bit more difficult.
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In my opinion proportions are really important when drawing. It can make such a different if the proportions are just slightly off. Especially if you are aiming for your drawing to look like a certain person.
It can be a very time consuming part of the drawing process but it will pay off afterwards if you take your time to do it.

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Drawing is seeing
An old teacher of mine once said that drawing is seeing. You might know the feeling of having an image of some kind in your head but you just not able to get it down to paper. It's about learning to see the object, in order to understand them and draw them. I think that's what makes it hard to get a drawing to look exactly what we want them to at times.
I have found in time it gets easier. The more you draw things the better you get at them and you memories the basics naturally.

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