Shaking cocktails without bar equipment

I've been able to shake and strain cocktails with two cups pretty easily. Ideally one cup should be slightly smaller than the other, though this isn't absolutely necessary if you're careful.

Put the open ends together and shake as necessary (if they're slightly different sizes as described, you can get a real nice seal and shake vigorously). Now, place one cup on the counter, and the other on top of it upside down, ideally still "inside" the other one.

You want to tilt the top cup about 5-10 degrees to the side, which will "break the seal" between the two cups and leave a very small opening. That's where you'll strain through. Now all you have to do is tilt the entire contraption 90 degrees and pour!

This is more easily explained with images...I will try to find or make one for you ASAP.

edit: Couldn't find a good example, so made one for you with what I found around the office:

There are several comments here recommending glasses or glass jars as shakers. As a bartender I can tell you that that is a very bad idea. Most glasses and glass jars are not tempered. As such, when you shake them violently with ice in them and they chill rapidly, they can and very often will break.

Bartenders you see using Boston Shakers (metal and glass two-part shakers) are using a tempered glass that is resistant to breaking.

If you can't shake your cocktails because you lack a good shaker, then your best bet is to modify your approach and try tossing your drink instead between two tin cups or used soup cans.

Put your ingredients into one can without ice and then pour from one can to the other with as much distance between the cans as possible without spilling. This will create the aeration that you're trying to achieve by shaking (aeration is the actual reason you shake a drink, and is generally only a goal in drinks with citrus or cream - spiritus drinks are generally stirred). Do this serveral times and then add ice and roll the drink back and forth between the cans to chill it.

You want to do that in two steps because with ice in the cans you won't be able to pour far enough to get optimal aeration, but without the ice, your drink won't be cold.

It's really not that hard. The purpose of a shaker is to get the alcohol really cold quickly.

I like martinis a lot, but find them a pain to mix. My solution has been just to keep the Gin in the freezer. Pour a dash of vermouth in the glass, and top with ice cold gin, instant martini.

Otherwise, I would just use a pint glass as a shaker. Cover it with a clean piece of cardboard or a coaster and give it a shake.

Most of the straining can be done with a fork on the edge of a glass.

I use an empty jar, overall easier to open than a frozen shaker:

and then a small drainer to prevent the ice from getting into the cocktail. It comes to great amusement to the guests as well :)

I use a travel mug! You can shake all you want, and the lid acts as a great strainer.

Because glass can break easily during shaking, I prefer upcycling one-way plastic bottles with a wide neck: filling is easy, even ice cubes can be filled in. Once you're done with shaking, dispose the bottle.

Specifically, I'm talking about these bottles:

There's always one available that is empty or at least almost empty and ready for upcycling!

For straining, simply use your tea strainer.

I use a workout shaker. It is the same one that you use to mix your protein shakes with, they have the little springy ball. It works well for me.