SAL or economy air (sarubin/SAL便) is a cheaper alternative to airmail if you are posting heavier items up to 5kg. It combines surface delivery and airmail making it slower than airmail but also cheaper.
Here is a list of rates (inEnglish) from the JP site:
For parcels that weigh 2kg or less, don't forget that the SMALL PACKET rate (小包） will be considerably cheaper than INTERNATIONAL PARCEL or EMS and can be sent either airmail or SAL. If you are sending something as a small packet, you'll need to fill out a 'green label' or CN22 customs declaration. This declaration is included in the address labels of International Parcel or EMS. I have found that some post offices don't mention small packet as a postal option and will send everything by international rate, which costs a lot more.
Small packets have quite strict size limits:
Small packets can be registered for an extra 410 yen. Using the small packet rate with registration and airmail will reach most destinations at about the same time as EMS. There is no track-and-trace, but it will have a number and be a signed delivery.
Happy packing and posting in 2010!