Guidelines

1. Beat placement should follow the time-distance equality rule (using distance snap while placing beats enforces this). This means that you shouldn't have some fast notes with a huge distance between them followed by closely-placed (but slow) notes. This gets really confusing if the player isn't the person who designed the beatmap! Having some logic to the beat placement should be one of your goals.
2. Avoid placing hitobjects under the HP/life bar at the top of the screen, as this can greatly impair the visibility of notes and approach circles to the player. It doesn't hurt to use the default skin to check this!
3. Try to use the same tickrate on every difficulty as it is a property of the music rather than the mapping. Using high tickrates to increase score/combo/difficulty is senseless.
4. Use new combos often and intelligently. Using a new combo every few beats wouldn't make sense, but you also want to avoid long combos that continue for many measures.
5. Slider speed changes should have corresponding prior cues. These cues may include changes in the music, changes in note density, or visible slider ticks, and should allow for the speed change to be expected before the slider begins. Large, unexpected changes in slider velocity damage the readability and playing experience of the map.
6. Generally, a spinner and the next object after a spinner should both have New Combo markers. You can remove one of the New Combos if the spinner belongs to the same musical thought as some objects around it [1] [2], but use sparingly.
7. Try to have at least one spinner in each difficulty to create variety in the map and fluctuation among scores. However, if a spinner just doesn't fit anywhere in the song, then there's no need to force one.
8. Make sure that your spinners are a reasonable length. Aim for a maximum spinning time of about 5-7 seconds; any longer can cause the player's wrist to get sore. If your spinner must be longer, then check with a BN to see if it's acceptable.
9. Try to avoid using hitobjects directly after spinners (especially on Easy/Normal difficulties). Spinners are sometimes the hardest element for players, and having a hitcircle or slider half a beat (or even a beat) after a spinner will commonly result in frustration and a broken combo. Hitobjects directly after spinners are fair game for Hard/Insane difficulties, but try to give a nice pause for Easy/Normal difficulties.
10. Use an even balance of sliders and hitcircles instead of focusing on one or the other to give the map variety and keep it entertaining for the player.
11. Avoid setting stack leniency to a point where you have perfectly overlapping consecutive hit objects. This setting is used if stacking is damaging to patterns that exist in a map. It is usually a good idea to keep it enabled.
12. Manual stacking is acceptable as long as it's still readable while playing.
13. Avoid covering up objects with hitbursts, slider tracks, and other objects, especially in easier difficulties. Obscuring objects can make them difficult to read. There can be some exceptions where patterns have a clear and logical path and the timing for these objects can be told easily.
14. All hitcircles and the starting point of any sliders should be snapped to a beat in the music. Slider end points should be snapped to a beat in the music, or the end of a prolonged note where appropriate. Adding hitobjects where there is no musical cue to justify them can result in confusing patterns which are difficult to follow and incongruent with the backing track.