Due to my current job and connections, I've had many instances where I've met and got to know many professionals. Over time I've asked them all about how to get a job in the industry working for one of the larger comics companies and very often they give a good list of some the best advice you could ever ask for.
First practise, what the companies are looking for are not people who can draw extremely well but people who can tell a story through images and visual narration. So stop drawing detailed pictures of characters and start trying to draw a comic, be it from a script or try re-imagining an existing comic. Obviously standard advice such as constanly practise apply as well, but if you want a job as a comic artist it's all about how you can tell a story.
Next is applying when it comes to sending your portfolio, don't do it digitally. Often editors will just ignore any emails as they don't have time to check the dozens of applications they get via this method. You're best bets are to either physcially send it out or if you can go to portfolio viewings as these are the times when editors are in the right mind to critically review your work for hire. Don't be dishearten by what one editor says though as often they're looking for work that fits their opinion of comic art. Take any critism they give, keep it in mind, adjust your work accordingly and keep applying.
Networking and faux pas's. The comic industry is much smaller than many people believe, and gossip/reputations are built and destroyed very easily, be careful when taking about comic work with another artist as the wrong phrasing could mean trouble down the line and result in people not wanting to work with you. At networking events it's always good to stay calm, friendly but not boisterous, if theres a bar remember to keep track of your limits.
When it comes to wanting to work independantly such as selling your own comics the biggest mistake I've seen is people charging too much for their product. It's understandable to want to make something look professional but you're not, try make your production costs as cheap as possible to sell your product for a minimal fee. Rarely have I seen an indy comic sell for more than £2.50, currently the average cost for a comic published by professionals is £2.20, unless your comic is alot better than that theres not way it's going to sell. Avoid making graphic novels by yourself as the feasibilty of you getting any money back is slim to none. If you're charging more than £2 a comic you've got to make yours different some how, add a feature such as an inbuilt game to make it worth the purchase.
My personal opinion on publishing comics independently is make it interesting to your target market, make it stand out by maybe spending extra for a colour cover or a free toy etc or a personalised advertising campaign, keep interiors black and white until you can afford full colour, keep printing costs cheap. Many people want to earn back their money but thats just not possible, cheaper the printing costs the less you're going to lose, ideally you're comic should cost no more than £1.50. Be prepared to make a loss, if your work is good enough word will spread about your comic and slowly you'll become recognised within circles, fame is more important than cost as if you're a recognised artist you may begin to recieve work from people higher and higher up the ladder in the comic industry.
All of this advice should be inexpensible for anybody looking to get into the comics industry and I personally plan on following these rules should I decide thats the area I want to work in