The best advice I can give to the average unregistered player is to get registered! There is no advantages to playing unregistered and you can only do yourself favours by fronting up with the pathetic amount of cash that getting registered requires (ie. You will immediately start winning more games against decent players). And when I say pathetic amount I mean it. Even I, who has never had a job in his entire life and basically lives off the generosity of the Australian government :(, could afford to register myself. That is how extremely small a sum it is. Only $24 US dollars for the regular registration (the cost is more if you have to send an International Money Order from Australia to Canada like I did!). If you have any kind of job, get pocket money or anything like that and intend to continue to play this game, then you should be ashamed if you fail to register yourself.
Now that I've finished ranting :), on to the actual tactics. In fact, the best thing that can be said of unregistered tactics is that they are merely a subset of registered tactics. Unfortunately for the unregistered player, this subset contains the worst tactics available to the registered player and not the best. The main difference between registered and unregistered tactics is that an unregistered player should never buy any universities until it is absolutely necessary they do so. These necessities are:
If you follow these guidelines in purchasing your universities, you probably will not waste much more cash than utterly necessary and may just pull of a win against a half-decent registered player. On a moderate to small map anyhow... On a very large map, you will find yourself being forced to freight your units in from long distances to fight in an ongoing battle and will probably be pushed back by the superior mobility of a registered players forces (because they can have their new units appear instantly at the front and you will lose the ability to produce units as the cities containing the relevant universities are lost unless you purchase more. Which will put you behind again of course...).
Also due to that need for technology, an unregistered player must be constantly on the search for the oppositions own universities. They will probably be higher technology than the unregistered players own and they will also be within the opponents own empire, making them almost as good as a factory (sure you can only build one tech, but if you find say the Panther and the Grizzli Universities close to one another, you can really go burn a hole instead your enemies empire). Which brings me to the topic of maps for unregistered players. The correct choice of maps is very useful in helping you find enemy Universities. The kind of maps we are talking about here are the "pod" maps. The best examples of these maps are Airwar, Airwar 2, Antartica and RiverRace. Other maps follow the pod style to a certain extent, but on these maps, the placement of universities is at its most predictable. Then there are the maps on which you probably should not play at all. These are the big maps. The ones were you don't have a hope in hell of finding enemy universities amongst the hundreds of cities they own (and if they know what they are doing, they are on the other side of their empire from you anyhow....). You also wind up wasting thousands of dollars from buying your technologies over and over again (when you get in attack range of an opponents base usually).
The final difference between unregistered play and registered is that you must protect your empire even more vigilantly than that of the registered player. If a registered player loses a university, he has only allowed the opposition to gain a place from which to buy some units. But the importance of that same university to an unregistered player is vastly more. The unregistered player loses his ability to produce the unit from that university (at least in that area if he has more than one such university) until such time as he is capable of recapturing that university or is able to build a new one of the same type. If the university that is lost is of a high level, then this may very well be impossible and cost the unregistered player the game! To protect against this, the unregistered player must make every effort to ensure that he does not lose these very valuable cities. If this means that he must fortify and put at least one Porcupine in every one of these cities, then that is what he must do!
And that is basically it. An unregistered player must plan very carefully to avoid wasting too much more money than the registered player and must always be ahead of his enemies in every way if he expects to succeed.
NOTE: Strangely enough, some people actually find this added difficulty attractive and so you may well find quite experienced players playing under unregistered aliases. I won't pretend to sympathise with that desire. In any case, be warned that there is a subculture of experienced unregistered players who will take you down a peg or two if you get lazy. Actually, I just happen to have a little bit of strategy (?) written by one (ex-)Colonel Dion... It tends to advocate game styles that I wouldn't normally consider playing, so I figure I'll share it with you.
Dion's Unregistered Strategy Guide