Leather Crafting 101 - You gotta get good leather.
Period. Thats it. If you want to create quality leather products that last, hold up against wear and look good (something you'd be proud to call your creation) then you cannot skimp on leather quality.
I know it seems like common sense. "Of course if you want quality leather products you should buy quality leather to make them from," you think indignantly, but you'd be surprised at how many folks get lured in by the Sale Bin at Tandy Leather (or whatever your local leather hobbyist supplier is). Remember that the things found in a 'Sale Bin' are there for a reason -- it might be an especially low quality piece out of their latest tannery run shipment and so they know they can't sell at full price OR its been there a while and they couldn't move it earlier because upon inspection people saw that it wasn't worth what they were asking. So, they roll it up, tie it (making it harder to inspect) and throw it in the Sale Bin. I'm not saying you can't find decent stuff in the Sale Bin, I'm just saying that generally speaking, the Sale Bin is not where you want to go for quality product.
I also understand that its tempting to go cheap when you're first starting out. You figure that you don't know what you're doing yet, you're just learning, you're experimenting...seems wasteful and expensive to buy good leather to mess up on. However, good quality leather behaves differently than poor quality leather and so your learning experience will be consequently different. Good quality leather is pliable but firm, the grain is tight, it soaks water and dye evenly and it cuts and carves cleanly. Poor quality leather is loose in texture and grain, it will soak water and dye unevenly and cause blotching (due to areas of loose grain). Poor quality leather will also cut and carve differently; again, due to areas of loose grain, stretch marks, blemishes or scars.
So I would suggest that you learn and experiment on decent quality leather. In the long run you'll hone your skills finer and have an easier time getting there. Consider this - learning a new skill can be hard enough, why make things harder on yourself with material that works against you? You don't have to start out on Hermann Oak, but instead browse through the available leather at Tandy and inspect every inch of every piece before purchasing. That way, you can still get a medium to good quality side without breaking the bank.
So, what qualities should you be looking for?
Tight Grain
As you move and bend the leather in your hands, it should feel uniformly firm but still pliable. There shouldn't be any large areas of looseness or "floppiness" that would indicate a loose grain that comes from inconsistent/uneven tanning.
Notice in the image above: These two strap pieces were just cut. You can tell the different in quality in how cleanly a quality piece of leather will cut. The strap piece on top has a looser grain and a 'stringy' texture on the surface and along the recently cut edge, while the strap piece on bottom has a uniform texture and much tighter grain.
Even Weight/Thickness
Unless its really bad, the weight and thickness is not usually visibly noticeable. Run the piece of leather you are considering between your fingers - check along the edges and to what degree you can, through the center. Feel for inconsistencies in thickness. No piece of leather is perfect, but check for noticeable or extreme changes - its usually a mark of poor processing. Also, it bears mentioning that the leather supplier is selling this to you by weight and if the piece you are considering has large inconsistencies, then you're probably not getting your money's worth.
Smooth Velvety Flesh Side
The flesh side (the side that was on the "inside" of the animal, not the hair side) should be of uniform texture and have no large areas of "shredded" looking fiber. It should be velvety and soft to the touch and generally it shouldn't feel "dry" or rough like sandpaper.
Notice how the above image shows a smooth and largely uniform texture. 
This image shows a particularly poor quality flesh side - showing dry and loose grain. This "shredded" appearance is usually a good indicator of poor quality.
Few Scars, Blemishes and Stretch Marks
Blemishes and marks are not entirely avoidable - perfection shouldn't be expected and often the natural look of the hide and all its history is desirable. This was a living breathing animal and, like you or I, has a life history that shows across its skin. That being said, while a few blemishes, scars or marks are to be expected, you do want to find a piece that is largely without trouble spots. Remember that, depending on the size and type of blemish, these areas can render large pieces of your leather unusable.
No Wrinkles or Warping
When you bend the grain side back does it remain mostly smooth and tight or does it wrinkle like cardstock paper? If it wrinkles badly, its another sign of loose grain and poor tanning. You want to be able to bend back most of the piece you're considering and for it to stay relatively smooth and without warping or bad wrinkling. 
Notice how as I bend this piece the leather remains smooth at the flexed points.
The wrinkles on this piece of leather look very similar to cardstock paper when you try to roll it. This is a mark of loose grain and you can feel the difference.
Hopefully, for those of you just beginning, those simple pointers will help you in your leather selection. Maybe it'll save you a little trouble down the line.
Happy crafting!