• What is causing the illegal immigration
• What can we do to prevent this
The first one is actually a two-fold answer in my mind. First, the reason why there is such a massive influx in people is the difference in socio-economic levels between either side of the fence. There is not much you can do about that that will have a short or mid-term impact on the (illegal) immigration. The other reason is the MASSIVE wait-list for visas for immigrants from Mexico. It can take up to 15 years before your (approved!!!) petition is eligible for an actual visa. Since there is a cap on the amount of visas issued and the demand FAR outreaches the supply you see this extreme waiting times. That is the root-cause of the whole issue. Whatever happened to the phrase on the Statue of Liberty:
Give me your tired, your poor, your huddled masses yearning to breathe free, the wretched refuse of your teeming shore. Send these, the homeless, tempest-tost to me, I lift my lamp beside the golden door.Any artificial barrier (whether it is a physical wall or a 15-year paperwork process) is not going to have the desired effect. That is trying to deal with the issues whereas this needs a fix on the root problem.
So how do we prevent illegal immigration, what can we do to prevent it? Here is my idea for a major step forward. I'm not saying it solves it completely, but you'll most likely reduce the problem significantly. If you would have a system where an immigrant can petition for a visa and either be granted (read: issued) or denied within a reasonable amount of time (I'd say about 12 months) at a cost far less than the current human traffickers charge you'll see a dramatic reduction in the illegal crossing of this nations borders.
Why would you consider such a program? There is so much work available for them (yes, even in this state of the economy) that the current cap is unrealistic. Consider that if all illegal immigrants would simply stop working it would have a major paralyzing effect on the already fragile US economy. That is pretty much a fact. Just imagine that if the US census bureau estimates there are about 12 million illegal immigrants in the USA, so even if only 10% of them would work you'd have 1.2 million people on strike… Not even the UAW can match that… Therefore any politician that wants to rid the USA of illegal immigration is talking pure politics since they all know that it's entirely unfeasible and even undesirable to do so.
Then there is the additional issue that for the legal immigrants there is a large demand for seasonal workers that cannot be filled due to the way that the system works. You can only apply for seasonable visas a maximum of 90 days in advance. The 'season' runs from October to September, but in reality within 1 to 2 DAYS the (capped) amount of visas available for those seasonal workers is depleted. So any industry that needs workers from February on (and I'm not talking about the IT industry here…) can never use those seasonal workers - and since the existing workforce finds those jobs like crab fishing, fruit harvesting, etc… undesirable - the only solution is to use the 'undocumented' workforce.
However, if - for political reasons only - you create those artificial barriers around immigration that have no regards for the existing situation and the economical and demographical supply and demand you'll never create a feasible solution and all you'll get is political populism and history gives us a very clear answer on the outcome of that: it doesn't work (and that is the 'softest' description available…)