Here are three simple tips I find useful.
1) Sleeping in economy seats is tough because of limited space and recline. Consider spending up a little bit for more comfort. Even Economy Plus or a choice seat can be a meaningful improvement.
2) Choosing a window seat or (in two-aisle aircraft) a middle-middle seat means nobody will disturb you to get up to visit the lavatory.
3) A comfortable meal and a bit of alcohol help with getting to sleep. But don't overdo it-- too much food or booze degrades the quality of sleep and causes other problems, too.