I have an old house with lath and plaster walls. When I moved in a year ago I fixed dozens of cracks in the plaster with spackling, but many of them have opened back up again. What is the best way to solve this with a more permanent solution?
Spackling's best use is to fill holes made by pictures, curtain holders, etc. To fix cracks in plaster, use Plaster of Paris or Durabond, not spackling compound.
Cracks are caused by moving or shifting, so one needs to make sure the existing plaster and corresponding substrate (in this case the lath) are solid.
A simple test is on both sides of the crack in several locations gently push the plaster. If there is movement, additional repairs may be required beyond a simple crack fix. For example, one may need to replace the lath or substrate. A simple crack may also turn into a large area where the plaster needs to be re-attached to the substrate. This is the case where the crack is hiding a larger issue of crumbling or decaying plaster and will need a larger scale fix.
I would advise doing these tests on all your cracks to determine the whether the plaster is just cracked or is suffering from bigger and more labor intensive issues.
If the plaster and substrate is solid, then to fix the crack do the following.
Plaster is hard to work work with and requires a skilled laborer to apply. So anything beyond a simple crack fix, I would go to a professional.
The plaster is likely cracking due to movement/shifting/settling in your house. If you want to keep the plaster, the only real solution is to correct the root problem.
Now, usually an old house would have already settled, so there might be a bigger question/problem as to why it is continuing to do so. Not to sound any alarms, but you should investigate this. Are there any new cracks in the foundation? Water in the basement? Sagging floors?
As for just fixing the problem at hand, you can either continue to patch it as it occurs, or as Tester101 said in the comments, you can replace the plaster with drywall since it is not as brittle.
In your case (old house), the house has likely already finished settling. Settling combined with the years of service the house has provided, may have caused some of the plaster and lath to become separated. Once this occurs cracks will form, and will be nearly impossible to cover. The product mentioned in the article, actually reattaches the plaster using glue. Once that is complete, the cracks can be repaired using a layer of Spackle.
If you're feeling up to it, don't mind a huge mess, and have the money to burn. You could always rip down the plaster and lath, and replace it with drywall.
Make sure you inspect the foundation and support structure before you do anything else. In some cases cracks can be formed because of underlying structural defects/damage, so you'll want to rule that out before you try to fix the cracks.