I've been watching a lot of Richard Dawkins' stuff on youtube (just google "Richard Dawkins youtube" and enjoy). I don't think that any of his powerful arguments moved me much from my current positions, but I did get very curious about the resistance he faced when talking about the concept of evolution. As a scientist, I believe in evolution. In fact, I had thought the debate about teaching evolution in schools was a knee-jerk reaction by narrow-minded folks.
I was totally wrong. While watching the Dawkins videos, and while looking at data I found on Wikipedia and other places, I realized that I was totally unprepared for any sort of, say, dinner debate with those who have problems with evolution. People against evolution can be articulate and highly intelligent. Their arguments are well thought out, and they have a support system of electronic media that provides them with lots of usable data. Do not underestimate them. Your high-school level, half-assed explanations will not fly and will be shot out of the sky like geese in hunting season. Lots of them are also really nice people, who volunteer, give generously and have strong families. If your plan was to use biting sarcasm to buttress your weak arguments, you'll appear rude and caustic, which will probably scare away any neutral and/or undecided listeners.
If you like science, get educated and figure out how to make good and clear arguments to support evolution. I've put together a strategy below, but this is just my take. Go ahead and make your own.
First and foremost, exclusively use the phrase "the idea of evolution". Avoid saying "theory" (which it is, technically) or "fact" (which it also is, as far as I am concerned).
Secondly, say that the idea of evolution is supported by an overwhelming set of circumstantial evidence. In this way, you acknowledge that no one can actually go back in time and see evolution for themselves. In that weak sense, we infer evolution from the evidence we have collected. However, you end strongly by saying that almost every fact discovered in biology and allied fields is evidence of evolution. The amount of evidence is truly staggering.
Thirdly, even though you believe in evolution, you wouldn't want to live in a society ruled by natural selection. It would be a painful place to live: it would be a place ruled by jungle law. Just as we can use our minds to understand evolution, we can use our minds to build a society not based on "survival of the fittest".
Fourthly, you ask for few seconds and hit home a small set of points:
a) Fossils: The process that forms a fossil is a lucky process and does not happen often. So the fossil record is incomplete. However, no fossil has ever been found that contradicts evolution. In addition, many "transition" species have been found like Archaeopteryx and the series of humanoid fossils. Although we'd like more fossils, all of those that exist support evolution.
b) Geographical distribution and environmental pressure: The same type of animal, in two close but geographically distinct regions (like islands vs. the mainland) can have quite different characteristics that seem suited to the environment. It suggests animals moved from one environment to the other (say from the mainland to the islands) and evolved. Furthermore, unrelated species display the same feature when the environment is similar.
c) Microevolution and Speciation: Changes in animal shape, behavior and other aspects can happen within human lifetimes (for bacteria and plants like roses) and also over human history (for domesticated animals such as dogs and cats). So you can infer that over even longer periods of time, such changes could happen (directed by the environment) and result in speciation. In addition, many instances of speciation have been observed.
d) Vestigial organs and Embryos: There are useless organs in animals, like the appendix in humans, that appear to have been useful in earlier stages of our evolution. Embryos of different organisms are also interesting. While concrete theories relating embryos and evolution are no longer accepted (so be careful if you mention that human embryos develop gills which later then disappear), there is modern theory on development and evolution.
e) Genetic material: Evolution is supported by DNA evidence. For example, if you say that humans are closer to apes than to snails, then the number of genes in our DNA will match with apes more than with snails. In this way, you can build a tree of life which correlates well with the fossil record and geographic distribution. Remember though, genetic material relates our cousins (species alive today) not our ancestors (species found through fossils).
f) Evolution as a tool: We can simulate evolution on a computer. This can be educational, to show how artificial species change. It can also be useful for industries and to solve real problems: for example in genetic programming. Since evolution can be a useful tool in technology, it makes sense that nature, over long periods, could use the same tool to make complicated biological entities.
Finally, you can end by the icing on the cake. Of course, this is the fact that no other idea in science, or any other field, explains all the evidence in a simple and logical manner. There is simply no other competing set of logical arguments. That last bit about being logical is critical. You can always come up with a magical idea, where a magical entity creates fossils and animals with vestigial organs just because it feels like it. Its important to know that anyone can retreat into such a superstition-based argument, and that these ideas explain the evidence, but not in a logically and intellectually satisfying way.