If symptoms indicate, be on the lookout for any of these ski tuning issues that may be a contributing factor.
Dull edges: Probably the most obvious ski tuning issue and the easiest fix. While dull or burred up edges won't matter much in soft snow. If you are having trouble gripping on hard or icy snow. This is the first thing to asses.
The old finger nail test is a pretty good indicator. Can the edges easily scrape off a bit of finger nail? check in several spots along the ski's edges. If you are skiing on hard snow frequently it will dull your edges faster and you'll need to tune them more often.
Base Edge Bevel: This is the angle of the bottom part of the edge adjacent to the ski's base. A larger degree of base edge bevel will make it easier to release the edges and pivot or skid the ski. Less base edge bevel allows for better grip but make it more difficult to release. A 'true bar' can be used to determine approximately how much base bevel you have. Between 0.75 and 1.0 degree is a typical bevel for all mountain recreational skiing. Racers tend to go with a little less (or even no bevel) especially for the technical events such as slalom.
Side Edge Bevel: The angle of the side part of the edge can be increased for a sharper edge but at the cost of edge durability and release capabilities. For all mountain skiing a side edge bevel of 1.0 to 2.0 degrees is average. Perhaps up to 3.0 degrees if you often ski on very firm snow. Some elite slalom skiers may go as high as 5.0 degrees but that could be dangerously grippy for most.
Burred Edges: If you edges have minuscule bits of metal protruding making them rough and serrated they glide don't as smoothly through the snow. They should be sharp but smooth.
Base High: If the ski is base convex in shape it can become difficult to grip. To assess for this run a true bar or straight edge down the base of the ski and look for light shining through underneath it towards the edges.
Railed: If the ski base is concave in shape it can become overly grippy and be difficult to release the edges or pivot the ski. Again run a true bar down the base of the ski and look for light shining through towards the middle of the base.
Base Texture: Base texture is that funny pattern etched into the base material of your skis. It helps the skis glide. Basically as your skis run along the surface of the snow they create a little slick of water to glide on and this texture allows that water to escape so it doesn't create a suction cup effect. Different patterns work better in different conditions with deeper grooves typically working better in wetter snow. If you aren't gliding well make sure your skis have some base texture.
Base Damage: Obviously hitting rocks that take big chunks out of the base will create issues with glide as well. In most cases a little P-tex will fix these in a snap.
Dry Bases: If your skis don't glide it's pretty hard to do anything else effectively. If your bases are dry (black bases will look appear whitish) then they desperately need a wax!
Weather Appropriate Wax: Different waxes work well in different conditions. Softer waxes tend to be made for warmer wetter conditions whereas harder waxes will help your skis glide better on colder snow. If it's one of those colder than usual or warmer than usual days at your local hill... this isn't to be overlooked.
How To Fix It: If you're a perfectionist (or need a race tune) and into DIY there are plenty of great tutorials out on the interweb explaining how to resolve most of these issues on your own. Here are a couple:
Or if you're lazy, like me, you can always just drop your skis off at your local technician and have them done professionally;)
Here's some links for waxing tips:
Recreational skiers who are into D.I.Y. and don't want to blow the bank getting set up will find this quick tuning tutorial from Guy at Alltracks to be very helpful.