so try to get better papers.
most drawer use bristol board paper (dunno the exact name someone else around hear maybe)
personaly I'm to lazy to check for it in my own city and so I use photo glossy paper for Inkjet printers the surface is ultra smooth and isn't sucking Ink at all(maybe a bit expensiv but it is allready around here).

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Wer nicht vom Meister lernt, lernt nie etwas.

Use a pen with good, solid ink flow- pens with bad ink flow look all crumbly when you draw with them.

Paper bleed can be a problem as Giam said.

Get longer, smoother lines by locking up your fingers around the pen, then ink from the arm, rather than the fingers or wrist.

Build up you lines gradually - it takes time, but if you start out with a thin pen line, then gradually add to it, you can thicken and smoothen it out as you go.

Try to distribute equal amounts or pressure on your pen as you ink a line.

a few general ideas,

when you alter the curves aim for a subtle S form.

To avoid pencil smudges use harder pencils (h2-h6).
But dont press them too hard or you will damage the paper, leaving a line that cant be erased (especial with mechanical pencils).

Use a kneaded eraser to get rid of smudges (also good to lighten helplines or lines that are to dark and interfer with your inks).

Keep your hands dry and/or lay another sheet of paper under your hand where it touches the drawing.

Instead of inking your pencil drawing direct, scan the pencils in(adjust the scan till the lines are light and change them to blue).
Then print them out, now ink the printed version(alternativly you can trace your original with a lightbox).

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Wer nicht vom Meister lernt, lernt nie etwas.

lol if you live in the UK check down your local WHSmiths for one of their pads (which will have a pencil sketch of a giraffe):3

There's a process I use to get real clean lineart and that is to use layout paper (or known as tracing paper) place the layout paper over your sketch, tape the corners with masking tape (so it won't tear the paper) and ink away. When you're done scan and you'll have one clean lineart :3 simple?