I've come up with a few tricks myself. I will ignore the sketching stage of this process, because this indeed becomes faster the better you get at it, so it's irrelevant. This also only applies to Adobe Photoshop, as I don't know any other program, so I apologize for mentioning tools that are Photoshop specific.
First, watch this video:
Result (finished in 45 minutes):

Confused? I'll explain.
1. Set your macros. You can record actions in Adobe Photoshop, by going to the Windows > Actions window. First make a new document, its size does not matter. Select the pen tool from the toolbar. Click somewhere on the screen, and click again to make a line with the pen tool. Then create a new set in the Actions window. Call it whatever you want. Then create a new layer. Set the function key to F2. Press record. Adobe photoshop is now recording your actions! Right click inside the document (while the pen tool is selected), and select 'remove path'. When you have done this, stop recording.
Create a new layer (use function keys F3 and F4 respectively) for these two actions as well:
- Right click and select fill path
- Set your brush size to 3px or 2px (2 will be very fine line-art), then right click and select Stroke Path (simulate pressure off!)
Now you have three macros that you can use while inking, as these are highly valuable and will save loads of time instead of right clicking and selecting 'remove' path or 'fill path' etc. It will cut your work down by a third.
2. After your macros are set, you can keep re-using them forever. So now it's time to prepare the sketch, or whatever it is that you do before you start to ink your drawing. Blow this image up about 3000 pixels in width. You want to have a huge drawing, because you can always downscale, but never upscale! If it's 300DPI then that's a plus as well, in case you ever feel like printing out a poster of your artwork.
3. Got the sketch, and got the macros? Create a new layer for the line-art, and start inking away! However. As you are inking (using the macros to cut down on time), do not delete the path for the line-art, and instead use it to fill the path up with a colour before you delete it (obviously the flat colour should be on a new layer!). You can see I did this in the video, as I was inking the face, I was also colouring the face at the same time. Loads of artists do line-art first, and then have to fumble around trying to colour the flats in, which can actually take up a lot of time to make sure you didn't skip any white spaces. Doing it like this means you are doing it at the same time as inking, and it will save you loads of time!
You will have to get used to filling in colours as you make your line-art, but you'll get better at it as you practice more.
4. Line-art done, and flats done as well? Time to shade! Create a new layer for each shade, and use the pen tool again to shade. As you can see in the video, I did NOT stay within the lines. I don't have to, and neither do you. Be as messy as you'd like. When you're done shading that specific part, go back to the flat layer, and go to Select > Load Selection and then again Select > Inverse. Click on your shading layer again, and hit delete on your keyboard. This should delete all that messy stuff that went out of the lines, leaving you with clean and simple shaded art.
5. Everything done? Merge all visible layers together (Layer > Merge visible), be sure not to merge it with any background. Save this image! Then resize the image to however big you want your sprite to be in-game, and save it again under a different name.
Working like this has halved my time working on sprites. I still spend a lot of work getting the sketch in the first place, but again, practice makes perfect. Using these tips, you should be able to ink and colour a lot faster than before.
The image below only took me 4 hours to ink, colour and shade:

So, do you have any tips on working faster?





