Things to try:

• Direction. - It is easy to get the collecting bug and go crazy buying tons of notes, spending hundreds of dollars on lots without the slightest notion as to what the notes are or where the collection is headed. In such cases, the notes are excitedly bought and regarded as interesting objects, but so much of what makes collecting worthwhile is missed. Before needless money is spent on impulse purchases, it is crucial to set some priorities. Over time, you collection will evolve quite a bit, so setting down some goals will produce an worthwhile collection that reflects your interests and preferences. Look and some banknotes, and see what you feel particularly drawn to. Are they the notes that come from countries that your ancestors were from? The notes that are from places you've always wanted to visit, but never did? The ones with historical significance in a particular era? The ones with beautiful landscapes? Interesting Roman architecture? Majestic fonts and seals? Fascinating life scenes? If you focus on these things, you'll avoid a sterile, unmoving collection.
• Gauging the value of a note. - For general purposes, the note is worth what someone is willing to pay for it. This can be influenced by endless personal factors. At any rate, though, finding out what others are selling the same note for can save lots of money.
• Circulated notes. - Lets pretend you have, sitting in front of you, two notes of the same denomination, from the same country, mint, and year. One has been through the washer and dryer, then to the fish market, then to some second-grader's class for show and tell. The other never went into circulation, and was promptly tucked into a non-PVC sleeve (using cotton gloves, of course). Obviously the one I just mentioned is worth more. Banknknotes have all undergone different treatment, and this reflects on tthe value of the note. From a collector's/investor's standpoint, this is why grading exists. Grading should not exist to limit a collection. I actually routinely go out of my way to find notes that have been around the block. Often circulated lots are much more unique, as they avoid the tendency to be uncirculated dealer stock. By allowing circulated notes, it is much easier to find notes that are older issues or that are from less commonly found countries. Estate lots can be a great sources for a decent variety of used but clean notes.
• Getting some background collection on your finds. - Especially if you are doing this as a family, this can be a blast. Grab an atlas, and get find some good online history refernces. Try to find out where Estonia is, or why Biafra isn't showing up anywhere. Learn the story behind all those German inflation notes, those Confederate War Bonds, and those French Revolutionary promisary notes (all of which can be obtained fairly inexpensively). In addition to a history boost, taking the time to look this stuff up can sidetrack aimless buying frenzies.

Things to avoid:

• The word "rare," especially when in all caps and followed by multiple exclamation points. - On eBay, I've noticed many sellers do this to draw attention to a listing. Some are experienced sellers looking to make an extra buck on inexpensive notes, while others may not really know much at all about the note, but assume that, because it is from some exotic-sounding place several decades ago, it has to be hard to find. At any rate, this buzzword has been applied to everything from inflation German notes to cheap uncirculated dealer lots to genuinely-impossible-to-find Panamanian notes. (As for the stuff that really, truly is rare, it's probably not the best launching point for a collection, anyways.) Approach listings with the word "rare" with caution. If you look at the notes in the lot and really feel like they would be meaningful additions to your collection, feel free to buy them. Do not, however, make any assumptions about rarity without some serious research.
• The word "colorful." - Once you venture outside of the US, notes are printed in a variety of different colors and sizes. To Americans, there can be a magnetic attrraction to this interesting colorful foriegn money. Honestly, though, it is hard to get an "uncolorful" lot. Like the word "rare," "colorful" is thrown out there to lure you in, but it has little reflectionon the intrisic value or virtue of the note.
• Buying dozens of "starter" lots. - For a first purchase, you can't go wrong with a nice grab bag of notes with a decent mix of countries, years, and denomintaions. Many dealers sell such lots, and they are a solid launching point, in my opinion. One must realize though, that buying several of these isn't going to round out a collection. There are only so many notes that can be obtained cheaply and efficiently enough to be included in inexpesive bulk lots. Once you hit this threshold, you get a ton of duplicates. You also limit the variety of countries you'll come in contact with.
• Sellers that gouge shipping. - They are out there. Banknotes are fairly easy to send in a save and economical fashion. Unless you are ordering internationally or getting insurance (which costs more) avoid the seller that is charging $10 to line the envelope with cardboard and lick a stamp. When choosing between two listings, be sure to calculate all costs before making a decision.
• Damaging the notes. - Though you don't need an undless supply of archival-grade goodies to keep your notes safe, some discretion is in order. Understand that exposing notes to sunlight, moisture, sticky fingers, cheap acidic holders, clutter, etc., will damage them. You will probably want nice sleeves for at least your most valuable notes notes (if not all of them). If at all possible, get a binder or shoebox with some envelops to keep things organized.