Edward Tufte has written several books about the visual display of information. They contain a wealth of detail about how to display data visually in a clear and honest manner - way more information than I can provide here.

Posters could be improved greatly by adhering to a few principles (see Tufte for details - and he'd likely scream to see these in a bullet list). Please keep in mind that this panel only skims the surface. There's much more that can be done!

1. As with everything else on a poster, your job is to communicate clearly and directly with your audience. You should work to eliminate anything that distracts from this.
2. First, be sure you plot the relationship you want to show. If you want to see the change in populations through time, the X-axis should be time and the Y-axis should be population. Nothing else makes sense.
3. The ratio of information ink to total ink in a graph should be as large (close to one) as possible. Information ink includes the markings on the graph that show the data. Anything else is just distracting - Tufte calls it "ChartJunk". In the above example, only the two lines show your data. The axes give the reader a sense of the range of your data. Everything else is ChartJunk: the grey background, the grid lines, the legend, and the overly-abundant axis labels.
4. Maintain graphic integrity - tell the truth about your data!
   - physical area on graph should be proportional to numbers represented
   - data should not be displayed out of context
   - explanatory material and labels should be included on the graph
   - time series with money should be inflation-adjusted

Here's a very simple example showing the relationship between two populations through time. The main point to be made is that the lynx (predator) and hare (prey) populations oscillate through time in a somewhat predictable manner. [ This is a classic example of predator-prey oscillations from the ecology literature, and has been interpreted as evidence of the regulation of prey populations by predators. ]

Straight out of Excel, here's what you get:

There's a lot of ink here that doesn't convey information relevant to the main point you're trying to make. Plus it's ugly.

1. Grey background: not only does it provide absolutely no information, it's also unsightly. After you remove it, you will likely have to darken some of the lines.
2. Grid lines: it's very unlikely that your audience cares about the exact values at each data point - it's the pattern that matters. The grid lines compete with the pattern you're trying to show.
3. Legend: it's taking up space that would be better spent on the graph
4. X-axis: The labeling between tick marks is confusing.
5. Axis scales: Because the pattern is the main focus, we don't need to have such finely detailed scales - just enough to provide a sense of the range of values plotted.
6. Y-axis label: Why make the reader tilt his or her head to read?
7. Legend: Why make the reader look back and forth between lines and legend? Just label the lines - then eliminate the legend.
8. Line types: Use color and line type to differentiate - this will help people who have color impaired vision, and also any grey-scale copies of the poster you make (as for handouts).

It's easy to make a graph that looks cleaner and has a higher ratio of information-to-total ink:

Without all the distractions, the relationship comes through loud and clear.