It's interesting how different style issues suddenly catch your attention.  Over the last few weeks I've noticed quite a few skirt slit problems.  Women have either a) tried to stitch them partially or completely closed, b) left in the 'X' stitch from the manufacturer (Ach!), or c) given up and let their slip show through.  Less than stylish options all.
 
1.  Always, ALWAYS remove the little 'X' stitch before you wear something the first time.  You'll find it on jacket/coat, sport coat, and blazer slits as well.  It's meant to be removed.  The manufacturer slips it in there to keep the slit neatly closed during transport and display.
2.  It is almost never possible to completely close a slit without causing the skirt to pull awkwardly, ruining the line and look.  You might be able to get away with it if you have less than an inch that needs to be closed.  But if you're thinking that thigh-high slit on a skirt you really want will look fine as soon as you stitch it closed, you're wrong. 

So what are your options?  If the slit is too high for your comfort, don't buy the skirt.  Find another.  If the slit isn't TOO too high, and you're up for a little fun, try this nifty trick...

Sew in a nice piece of silk lining!

I chose an animal print for my black pencil skirt because I like the unexpected flash when I walk. You can also choose a color to match your skirt but really, this is a perfect place to have a little fun.  How about a gorgeous purple or pink silk lining on a grey skirt?  Or a lovely black and white polka dot on a black skirt?  Red lining with a khaki skirt? Yes!  

Silk scarves that you don't wear anymore are great because you already have them, the edges are already finished, and silk is strong.  This is a high tension area of a skirt.

If you have reasonable sewing skills you can do this yourself or take it to a tailor.  I've only done this once and I'm still refining the best way to do it.  If any of you have tips to suggest, I welcome them.  In essence, what you're doing is as follows:

1. Lay the skirt flat with the slit on top.  Hold the top flap up, forming an 'L' and have someone measure the widest distance when the ends of the slit are pulled apart.
2. Measure a quarter inch from the bottom of the skirt (or where your hem starts if it looks better) to the top of the slit. 
   
3. Using these two measurements, cut a triangle from your scarf.  The tip will go at the top of the slit, the wide end at the bottom.  Get it? Before sewing, iron a fold into the silk with right sides together.  This way, the silk lining will lay folded and flat within the skirt after you've sewn it, as opposed to bunching together.  I found that stitching a small seam at that fold did an even better job. 
   
4. Don't forget, if you don't have a piece of silk with a finished edge, you'll need to do that.  Don't want this fancy little lining to have raw, fraying edges. 
   
5. With neat, tiny stitches in the same color as your skirt, sew the lining to the wrong side of your skirt and voila!
   

I quickly mocked up the insert I'll be putting into one of my denim skirts.  When Christmas and New Year's have quieted down I'll have time to create a detailed tutorial. 

A key point to remember: this works best with slits that overlap.  'V' slits, where the edges don't overlap at all, and may or may not even meet will still work, but the insert will always be seen.

Many women avoid certain skirts because of this problem.  Pencil skirts are ULTRA flattering on any body type.  With this simple solution you no longer have to avoid them or settle for letting your slip show.  This very easy and cheap fix will make the skirt in question infinitely more wearable.  Hope it works for you!