In hoodoo, the crossroads are a place of power. Hoodoo uses the four-way crossroads most often, those at (generally) right angles to one another. Another appropriate crossroads is a bridge that crosses directly over a straight run of water - working in the middle of the bridge is the same as working at a crossroads.

cat yronwode has a lot to say about crossroads in hoodoo here, including some interesting folklore and history.

The Dark Rider/Black Man at the Crossroads
Not unique to hoodoo, there are traditions of journeying to a crossroads to encounter a Black Man, Fetch, Dark Rider or Devil, to whom one sells one's soul in exchange for power or skill of some kind in many low magic traditions.

Appalachian hoodoo has a tradition of the Dark Rider. With this method, someone who is interested in learning conjure travels to the crossroads in the dead of night, with a talismanic item of some kind - a bone with meaningful associations, a High John the Conquerer root or something of the sort.

Once there, you wait for him, singing. There's no specific song to sing - you just sing whatever comes to mind, as long as you do so honestly. At some point, you'll be aware of either footsteps or hoofbeats behind you. At that moment, you stand stock still, not turning to look at him, and tell him what you want.

He doesn't say anything, but if you can feel him smile, he'll give you some of his mojo, putting it in that talisman you brought with you. It's sort of like the starter on a batch of sourdough - it's enough to get you started, but the real work of building it from there is all yours.

This practice - which is neither necessary nor even common among hoodoos - is a sort of initiation in hoodoo context. It is a beginning, and a declaration of intent.

Magic at the Crossroads
Lots of conjure is worked at a crossroads. Now, the ideal is the crossroads you encountered the Black Man at (assuming you did so). It's simply a place of power, with its own mojo that you can use as you need.

It isn't always possible to do work at a crossroads, however. Sometimes, you have to bring the crossroads to you. Many rootworkers will use a powder (sachet powders, or blowing powders) to lay down an X within a circle, and perform their magic on top of that.

Likewise, when laying a trick on a place with powders, it isn't uncommon for the powder to be applied in a quincux pattern - that is, four dots forming a square, with a fifth dot in the middle, like the 5-spot on a standard gambling die. Laying a hex on a business, blessing a house or any other trick that is laid on a place rather than a person is usually applied thusly.

Disposal of Tricks
A crossroads is also the ideal place for disposing of the materials left over after a trick has been worked all the way through. Bits of burnt paper, ash, candle stubs, jars of used honey and other similar ingredients should never be just thrown into the trash - you "throw away" the mojo you worked with them when you do so. Instead, bury them at a crossroads, so that the traffic at the crossroads "keeps the mojo traveling."

The Crossroads Keys
One trick that was shared with me was the creation of crossroads keys. These can be the talisman you bring to the crossroads when you meet the Black Man, if you like, or they can simply be something to provide you a connection when you've been working for a while.

Take two old skeleton keys, as alike in appearance as possible, and tie them together with a length of red ribbon. This ribbon should be measured from your heart to the outstretched wrist of your left hand. Tie the keys together, and carry them with you for a cycle of the moon.

Handle them often during this time, particularly if you journey to your crossroads during this time. Bits of your personal effects tied to them or dabbed on them are good, too - a bit of spittle before you sleep, or a lock of hair tied around both of them works perfectly.

Then, when you've carried them for long enough, take your keys (and a cutting instrument, like shears or a knife) down to the crossroads. Dig a hole in as exactly the middle of the crossroads as you can manage, and drop one of your keys down in there. Keeping the other key in your left hand, use your right hand to bury the key in the hole. Cut the ribbon that binds them, and then finish burying the key and ribbon in the hole, covering it well to prevent others from finding it.

Tie off the red ribbon on your key, and keep it. This is your "key to the crossroads." Think of it as the key that opens the door to that crossroads anywhere you are - when using it in your hoodoo work (even just kissing it and wearing it around your neck as you work), you are considered to be at that crossroads for the purpose of your mojo.

Best of luck!