What's this! You've barely begun planning for your primary career and I'm already talking about a second income? Well, yes, but I assure you that the two won't get in each other's way.
This particular second income is tax free, requires no training, can be earned at home (even while you sleep!) and has the power to improve your marriage. In fact, if I thought you'd believe me, I'd tell you that working hard at it can even increase your time for Torah and mitzvos.
Ok, you'll understand if I tell you just a bit more. It's about what you don't spend.
Your annual income is strictly limited: it was already set on Rosh Hashanah (see Beitza 16a and Rashi "Kol Mezonosuv"). But decisions about what to buy are up to you. If you're careless, you can fritter your income away in ways G-d doesn't approve and find yourself deep in a frightening hole. Add bank credit and the picture can quickly become very grim indeed.
Here it's worthwhile to quote at length the words of the Chofetz Chaim in his Be'ur Halacha at the beginning of ch. 529:

"One shouldn't artificially limit his Yom Tov expenses: however, during the rest of the year, a man must be conservative in his spending (Tur). And his (i.e., the Tur's) source for this is the Gemara 'all of a man's sustenance is set for him from Rosh Hashana etc.' Rashi (there) explains that 'one should take care not to spend too much because nothing will be added on to what was already set for you.' This is a powerful rebuke aimed at (the people of) our time many of whom, because of our many sins, transgress this. They don't properly consider how to manage their household expenses by avoiding excess. Many are the casualties of this evil behavior which, in the end, brings a man to theft and disgrace. ... Happy is he who stands firm, who pays no attention to these enticements and guides his household expenses in a calculated manner according to his wealth and not more."

It's certainly not my job to dictate how you spend your money nor to suggest which items are essential and which are the luxuries that you should avoid. Everyone has his own unique needs and expectations and, perhaps even more important, so does each of the partners in a marriage. But, nevertheless, what will follow are some general suggestions which can be applied where they seem most appropriate.
Try creating a budget. Itemize your expenses using headings like "food", "auto", "home", "vacation", "investments", "debt payments" etc. Then, beneath (or next to) each heading, add subheadings; the "food" heading, for instance, could be divided into "meat/fish", "fruit/vegetables", "restaurants", "junk food". Into each subheading, you could enter the cost of your regular monthly purchases (for once-a-year expenses, divide the total by twelve and enter that number).
Next, set firm and achievable goals. For instance, if your monthly expenses exceed your monthly income, you should probably aim to lower your expenses. Decide which items are expendable (or how much of a given item) and firmly commit yourself to your self-imposed restrictions.
If you've got debt, manage it intelligently. Learn what it takes to achieve and maintain a good credit rating. People with flawless reputations are the ones who can use the system the most efficiently. You should never, ever keep debt on a major credit card (their annual interest rates range from 17% to 28%!). But even a fairly low auto financing account can be beaten (and even a couple of percentage points can mean a difference of thousands of dollars over the lifetime of a large loan). Keep your eyes open for cheap credit - and, even more, for avoiding debt altogether!
Take care, however, to always carefully consider all the specific details of the financial dealings in which you become involved (or even seek professional advice). Not every strategy will work in every case.

Here are some other examples:
Do-it-yourself hair cuts will save (for each male member of the family) about $80/year (8 cuts/year @ $10/cut) - all for the one-time cost of a set of hair clippers ($25).
Following simple energy saving suggestions like using compact florescent light bulbs and water-efficient shower heads, turning the hot water tank marginally lower (an important safety consideration in any case) and installing (and properly using) a programmable thermostat can, together, save many hundreds a year.
Buy food in bulk whenever possible - team up with other families to gain greater buying leverage.
Time your trips to the gas station to take advantage of lower prices (fuel companies can regularly - and sometimes predictably - vary their prices by as much as 10%).

You might think that a few dollars here and there aren't all that significant. Remember, however, that saving five dollars on some small item once a week adds up to (about) $250 a year. Do that for ten different items and you're on your way to a decent second income.
There are, however, limits to all this. It should be obvious that driving an extra twenty minutes to save five dollars on cheese simply defeats the purpose. Besides the waste of gas (which can limit your savings), it's also an shameful waste of time: could you have been learning? Is this the best use of your time? Was the five bucks really worth it?
For more information on budgets and saving, you might contact your bank. Many financial institutions make very useful financial management advice available for free (check the web site of your bank).

---