There are two good examples that show that crema isnt necessarily proportional to tastiness. First, you can increase crema by using robusta and monsooned malabar. Such a blend has the potential to taste like rubber and wet socks, but it will probably have a ridiculous amount of crema that will persist for a long time. Second, coffee that is extracted quite fresh will often have a lot of crema that will quickly collapse. As the coffee ages and gas escapes, the crema will become more stable, but remember that as the gas escapes, so, too, do some of the more volatile and tasty components that you might want to drink in your espresso. The age of the coffee at which it delivers optimum extraction will depend on the roast profile, the packaging and the coffee itself.
There are a number of ways of increasing crema, but the key thing to consider is that everything that you do will affect the flavour of your espresso. Whether or not this is a better result depends entirely on your own subjective taste. As I mentioned above, you can add robusta or monsooned malabar. Dry processed brazilian coffees can also help you to build up crema. Obviously whether or not you want to do this depends on whether or not you like the flavour of the coffees that you can find. You can also age the coffee so that the crema that extracts contains less gas and is more stable. Some world barista champions have won the WBC using coffee that was three weeks or a month old specifically so that they would get better scores on crema. If you want to do this, how you store your coffee will be important. Vacuum packing immediately after roasting and nitrogen flushing will help. Whether or not you want to do this also depends on the type of espresso that you like. If you like heavy, rich espresso rich with sugar browning flavours like chocolate and nuts, this is probably an approach worthwhile trying, since those flavours seem to be due to heavier molecular weight compounds that will hang around. If you like espresso that has more interesting enzymatic flavours like fruit and floral flavours, this might not be such a good idea, seeing as those flavours seem to be due to lower molecular weight compounds that go away fairly quickly. If you want to preserve more of the flavour, possibly at the expense of decreasing body and increasing acidity, you can roast slightly lighter and store slightly longer. Whatever you decide to do, a useful exercise might be to actually taste the crema on a teaspoon.
Personally, I dont really care that much about crema. I remember that when Mark set up St Ali and Andrew set up Maling Room and we first started to learn how to use the temperature control on the Synessos, we were getting fairly low crema on espresso made with wet processed central american coffees. Synessos were the new kids on the block and some people posted online to the effect that "the emperor has no clothes." My response then was the same as it is now - Id choose better tasting espresso over better crema any day of the week.