When writing an email always remember that you may need to find it later. Use a subject that means something to you, as well as the recipient.
If a task is really important, why not email it to yourself, in addition to putting it on your 'to do' list. This can be particularly useful if you are away from your desk. Mark it as unread until it is actioned or no longer relevant. In fact, why not email your whole To Do list to yourself each day, so so it's there each time you go to your inbox, for you to be reminded and perhaps even memorise, without even trying?
If you've sent an email asking somebody to do something, copy yourself in, or copy it from your Sent folder to your Inbox or create an 'outstanding urgent issues' folder. If it's really urgent or important, mark it as unread until it's done - then delete it. You can easily keep track of all the things you're waiting for in this way. If there are a very large number of these 'outstanding' or 'reminder' emails, why not create a folder for those copies, called something like 'to do', urgent, 'outstanding', 'pending' or 'to chase asap'.
Keep up to date
If somebody has responded to an urgent or important request saying they have not yet actioned it, delete the copy of the original request from your inbox and keep their response as a more up-to-date reminder. You can instantly gain visibility of any given issue in your inbox. And because you return there often you are constantly reminded of outstanding issues. After a while, important outstanding issues become memorised.
Creating a folder for each of your current projects can be helpful. If you have a large number of projects, to keep your menu tidy you can put all the project folders in a folder called 'current projects'. If you send an email relating to a project, copy it from the sent folder to the project folder: make your important emails easier to find by having them in more than one place.
If you have a large number of emails, it can be particularly helpful to distribute these into folders labelled according to priority, such as 1, 2 and 3, or high, medium and low - or folders named according to the project, department or issue.
Keep your inbox for things you really need to know about.
Keep your inbox tidy by deleting any emails that do not require further action - and do not contain information that may be needed later. If they contain important information, copy them to a relevant folder (or to more than one folder if more than one is relevant). If they require action, leave them there and mark them as unread if they're urgent.