The most natural false lashes are usually individual lashes, but there are ways to make the strips look nearly undetectable as well. All it takes is a steady hand and some makeup to make either type blend in and accentuate your eyes.
Brands and Styles to Consider

In general, look for short, flared lashes. Many people find that the lashes look longer than they expected once they're on, so start with the shortest you can find and work your way up from there. Brands and styles to consider:

Ardell 65095 DuraLash Flare Short Individuals
MAC Cosmetics 7 Lash Strip
MAC Cosmetics 30 Lash Individuals
Ardell 65011 Invisibands Wispies
Ardell 65012 Demi Wispies Strip
Lash Strips Application Process

Getting a natural look with false lashes is easy to do, but be sure to practice your application a few times before you plan to wear them out just to get the hang of it.

Remove the Lashes Properly
Remove the lashes from the packaging by gently pressing down on the band with the pad of one finger.
Sliding them off of the package with light pressure will keep them from stretching out. Don't just grab one end and tug the lashes straight out of the package. Take each end between a thumb and forefinger and hold the lashes in an upside-down U-shape.
Alternate raising and lowering your hands about an inch to make the band flexible and more adaptable to your eye shape.
Preview Before Gluing
Before putting any glue on the lashes, lay the band on top of your natural lashes with the shortest side at the inner corner of the eye.
Trim the excess width from the outer corner, leaving the shortest lashes intact. Hold the lashes to your eye again and check the length.
If they still seem to be too long, trim the length by holding the lash band with tweezers and cutting the tips a little at a time, positioning them on your lids periodically to check your progress.
Customize Length
Make sure that you keep the lashes on the outer edge longer than the ones on the inner section; the length should taper off as the lashes approach the inner corner. It's important to cut the width before the length because you'll be cutting off the longest section of the lashes when you adjust the length to fit your eye.
If you wish, you can cut the lash band in half and use the inner half on the outer corner of the eye. These lashes are unlikely to be too long for the outer corner of the eye and will give an uplifted, natural cat eye effect.
Add the Correct Eyeliner
Once both sets of lashes have been customized for your eye size and the natural look you want, set them aside. Tightline the upper lash lines and then line the lids with a color that matches your false lashes. If you chose brown lashes, use a brownish black liner.
If you chose black lashes, use black liner. Using liner first will help fill in the gaps between your lashes and the skin, so even if your application isn't absolutely perfect, no one will know.
Time to Glue
Put a drop of lash adhesive on a dish or piece of foil. Holding the tips of the middle lashes with tweezers, drag the band through the glue. Be sure to coat the ends well so that they won't peel up throughout the course of the day.
Wait 15-30 seconds for the glue to become tacky.
Look down into a handheld mirror. Position the lashes at the base of your natural lashes and press down at the base with the flat side of the tweezers. Press the corners down, then gently press down from the inner to outer corner to ensure that the false lashes are attached across the width of the eye.
Once the glue has completely dried, use a gel or liquid liner to cover the top of the band and blend it into your lash line and the eyeliner you put down earlier. Use an eyelash curler to combine your natural lashes with the false ones and finish with a coat of mascara.
Individual Lashes
Individual lashes have a reputation for looking more natural than lash strips, but some consider them more difficult to apply. Some individual lashes are intended to be left on for up to two weeks, so if you're looking for an option that you don't have to apply day after day, these are a good bet. Just be sure to purchase the lash glue intended for individual lashes. It comes in a vial rather than a tube, and it can be clear or dark.

If you want to wear your lashes for days, make sure the eye area is free of makeup and moisturizer. With individuals, you won't need a liner first. Hold the tips of the lashes with tweezers and dip the base into a drop of lash glue. Let the glue get tacky and then apply it to the skin right above your lashes or between the lashes. Press down. Work from the outside in.

For the most natural look, stop halfway across the eye rather than going all the way to the inner corner. Once the glue has dried, curl your lashes and apply your eyeliner and mascara to blend everything together.

Getting the Natural Look with Falsies

The biggest tip for getting the most natural look is to err on the side of the shorter lengths, at least at first. With strips, you'll also need to trim the lash band to the appropriate width. Liner hides the bands and knots, so it will be necessary with false lashes even if you normally prefer not to wear it. It's completely possible to accent your eyes with natural-looking false lashes.