Weather systems are huge energy machines with a lot on inertia... it takes a huge amount of energy to shift these systems around. And if a weather system is tampered with without knowing where you're pulling it from, and what it'll grow into, you will likely unleash a tiger with more strength than wished for.
Weather-working has a long respected lineage in virtually every culture for just these reasons. A person (or subgroup) who works with weather can, by definition, pack a lotta punch. And a person who actually can pull these energy systems around reliably and safely is someone who is worthy of respect. (And a lot rarer than you would think. A lot of people who brag about their weather-working are really just full of hot air...)
Ok, with that disclaimer, there are some things you can safely try *without* spending your life studying the ways of weather-working. Do your meteorological homework first. Know what your "normal" local weather is like, and what the prevailing winds are. Know which direction the weather systems that usually bring rain to your area come from.
Next, since you probably don't have enough personal energy to build a cloud, you need to find a source of energy that can. I strongly suggest that you tap into the energy of your Deity of choice for this. Deity is strong enough to yank around the jet stream. If you have a close personal relationship with Deity, then you can tap into that. If not, you should be able to get the energy you need from Mother Earth. Wake Her up and explain your need. It's Her need, too.
You should also be incredibly grounded when doing any kind of weather-working. You will be channeling large amounts of energy, and a good solid, grounded connection to the Earth will help prevent you from turning yourself into a magical lightning rod. (No. That's not a fancy metaphor. If you're not absolutely confident in your ability to ground, don't mess around with the weather.)
Set yourself for a full ritual. This is a serious, desperate need for you, so give your ritual every trapping you can. Cast a full circle, dress in full ritual gear, wear your sacred jewelry, etc. You will want to direct the attention of Deity to your drought, and clearly visualize the end result (bountiful crops). If you simply visualize rain as you begin to channel vast amounts of energy, then you run the risk of causing flooding.
As a helpful aid in ritual, I strongly suggest incorporating a rhythm instrument of some kind. Many cultures use drums and/or rattles in rain summoning ceremonies, and it is commonly believed in American drum circles that a 4/4 rhythm will encourage the rains to come. (ONE-two-three-four, ONE-two-three-four) Use the force of the beat to carry your need upward. Cast your conscious will into the upper atmosphere, and trace your typical weather patterns for sources of moisture. Pull that weather toward you, and feel it's weight. It can be exhausting, so beware.
When you're done, ground yourself completely. Keep up your visualization of bountiful crops. A Rain Ceremony is often repeated every few days during drought (which is why it was often the sole job of a member of a tribal community.)
~Flame RavenHawk