Newer couches have feet that screw on, as at the bottom in the picture below.  If yours does, you can make a simple longer couch foot, shown below, out of 4x4 lumber 

Picture #1, 4x4 longer leg compared to original leg

When you’re done and the new longer feet are installed, the couch will sit higher, at whatever height you chose.  No more struggling to get out of the couch! 
Your couch will look like this:

Picture #2, Leg extension installed

It’s simple and costs less than $10.  The only materials are a special screw/bolt and a 4x4. 

To make the extended sofa feet (if your sofa has screw-on feet):

• Determine the current height of the couch seat and how much higher you want it to be.

• Unscrew a foot and take it to a hardware store.  Ask for a hanger bolt; these have wood thread on one end and machine thread on the other (on the left below with two nuts on the machine thread end).  Get one with the same machine thread as the sample foot you brought along, See picture 1. 

• From a lumber yard that has a radial arm saw that can cut a 4x4  in one pass, buy a 4x4 and have them cut 4 pieces the length of the original foot plus whatever added height you want; on my sofa, this was 8 ½ inches long.  Make sure the cuts are true and square, so it sits flat against floor & sofa!  Home Depot has both the hanger bolts and a big enough radial arm saw.

• Use a drill press to drill a true vertical hole in the end of each 4x4 to accept the wood screw end of the hanger bolt.  It takes a drill press to make sure the hole in perfectly vertical.

• As in the left-hand picture below, thread two nuts on the machine thread end of the hanger bolt and jam them against each other to lock them in place, then use the nuts to screw the hanger bolt into one end of the 4x4.  Unlock the nuts and remove them…ending up like the right-hand picture below.

• Remove all four original feet and screw on the extended feet made out of 4x4 lumber. 

Picture #3

Disclaimer: If this work is done in a workmanlike fashion, it will be reasonably strong.  However, you should realize that the longer the extension, the more leverage it can represent.  You should take care when moving the sofa; pick it up, don’t push it around!  If your extensions are long, you might consider reinforcing them with a strip of plywood between them to protect them from possible shear force.