Leg straps or clips (especially on the chain side) are the way to go. Not only does this keep the pant leg clean, but it prevents the cuff from chafing or hanging up on something.
You can improvise with a large rubber band, but the Velcro straps are readily available and very easy to get on/off. When not in use they can be wrapped around your seat post or some such.
What I think is the simplest (and one I do myself) is to just roll up your trouser legs. I usually roll mine up a few times to hold them in place and roll them pretty high. I do both legs so that neither of them can get caught in my water bottle holder. Ever since I started doing this, I have not once gotten grease on the outside of my trouser legs.
Shorter pants. Shorts or knickers will stay out of the way. In the winter, you can wear tights underneath. I love my wool tights so much that I wear them under my jeans during the day and at night as pajamas. They're expensive, though.
Metal pants clips. These are dirt-cheap, durable, and they work great. I usually only put one on my right leg, but in the winter it can be nice to have one on the left, so cold air doesn't blow in as much. I store one on my top tube, but if you're concerned about your paint job, you'll need another place. There are even some with built-in reflectors , but I haven't tried them yamaha dirt bikes parts
Velcro straps. They seem attractive at first, with bright colors, built-in reflectors, and sometimes even LED lighting. You can even buy bulk velcro at a hardware or crafts store for not much money. However, I find they work loose as my calves flex. Also, the time I tried the LED style, they broke within minutes.
Full chain guard. This is what I saw in Sweden and The Netherlands, where riders wore business suits and full-length dresses. In the USA, most riders worry about weight too much to consider these.
Tuck in to socks. This only works if your pants socks are long enough. As you ride it can still work loose. If you're not wearing socks today, obviously this won't work. But it's nice in that it doesn't require any extra equipment.
Roll up pants. Again, nice because it doesn't require special equipment, and even works on no-sock or short-sock days. I find I have to roll my pants up really, really far to keep them out of the chain, and they unroll as I ride. Meanwhile, cold air hits my delicate ankles.
Another alternative is using a "wax" chain lubricant. If you do some reading, you'll see that people have strong opinions about chain lubrication (do a search here for "wax"). I can see the reasoning behind most of the opinions, but I've personally had good luck with straight paraffin wax. White Lightening, Finish Line), or wax mixed with additives. The bottom line is that these wax-based lubricants are meant to harden and flake off with time, so they don't leave a chainring stain on your pants or leg.
Previously, I used straight paraffin wax (similar to this technique, but with no beeswax), but I've just started using a chain lubrication method involving graphite and wax. This approach seems quite good so far (only a few hundred miles).
If you do go with a chain wax, you'll definitely want to step up the frequency of reapplying in wet weather.
Simple. Cycling clothes on the commute and work clothes at work. The cycling clothes may or may not get greasy and there is no reason to care about it. However, the separated work clothes remain perfectly acceptable for the work environment.
Obviously, this may or may not work for everyone; but, I just change clothes. (Changing clothes simply requires a restroom stall or perhaps a company shower area for maybe 8 minutes at most.)
BTW - My current commute is up and over one big hill, and 2 smaller ones along the way. If it were all flat and easy, I'd just use trouser cuffs and ride in street clothes.