The more planning and the more thought you put into a spell - ALL of this is energy, not just the stuff you raise in circle while charging the candle - the more power it will have. And it's focused, directed energy - the mental energy you put into researching and selecting the right herbs, then searching for and obtaining them (or better still, growing and drying them), then measuring them out and actually making the candle yourself. making your own candles can often give the best results, because from the very onset of creating the candle, before you even say one word of power, you are building your spell.
Likewise with the oils - research and develop a nice blend of oils to add to your candle (any oils will be fine, but "candle making" oils tend to stink in my opinion. I prefer the real stuff.)
Scent is an incredibly powerful mental trigger, and I use it often in magic. I can recall with clarity my aunt's orchards in upstate New York when I smell burning leaves in the fall, because I used to run around her orchards with my brother in the autumn when I was a kid. Whenever I smell burning leaves, immediately, my mind conjures up that vivid and delightful nostalgic image.
I use this concept a lot with spells as well, because if I find scents that evoke certain emotions or mental states, or even physiological changes as certain scents (eucalpytus, say, for clearing sinuses). When I read from many sources that this scent is associated with this mental state or goal I wish to achieve, I make note of it and add it to my formula.
If it were my goal, say, to draw more customers to my web/graphic design business, I would want to put together a pleasing blend of scents of plants that smell like money; that is to say, scents which inspire me to think clearly and be a good communicator with my clients (air), which help me project a lot of charisma and energy (fire), which help open up my creative facilities (water), and to remind myself that I'm a master craftswoman at what I do (earth.) Though it is sometimes not at all appropriate to incorporate all of the elements in my blends, I often feel that my magic is more balanced when I do.
Magical candles can be beautiful works of art, each unique to the specific purpose for which you have created it - the form itself is also part of the spell. If you want to have fun with their coloring, melt crayons into the wax, using colors appropriate to your goal. Try melting some crayons in a sepaerate cup and pouring the colored wax into your candle after you've let the wax settle some, and you'll get some interesting two-tone drip patterns. And if you really want something odd and unique, put some ice cubes in a cardboard wax milk container (which you'll cut away afterward) and then pour your hot wax over them to get a beautiful lacy effect. Here's a good tutorial I found on this kind of candle-making.
As a sidenote, never, ever take what you read from just one source on this material. Lots of research and lots of cross-referencing will yield the best results. A lot of people have a lot of differing opinions on which scent is associated with which planet/mental state/goal/element/deity.. whatever.. and will list their own observations as a result. I've often heard dill associated with fire, but in my nose and mind, it's an air herb through and through. Ultimately, this is what matters most, though researching and comparing the wisdom of others is helpful to add to this.
In fact, doing a little bit of reading lead me to exactly this. A recent study has shown that a reaction to a scent can be programmed. If you were stung by a bunch of angry hornets while sniffing at some lavender, I can guarantee you, you'll never associate lavender with "calm" and "serenity" again. This is an impressive study, and useful reading to anyone doing this kind of magic. When you're working your magic to charge the candle, put a bit of the oil and/or herb blend to the side, then put the rest in your candle. Then, breathe deeply of the oil/herb blend you added while saying the incantation, carving the sigils, or however you are imbueing your candle with power. Link those words with that scent.
In my hometown of Memphis, TN, you can find herbs in bulk and oils for scenting candles at:
13 Florence St
Memphis, TN 38104-4123
1331 Madison Ave
Memphis, TN 38104-2325