The very hard ones are usually a combination of elimination and squares that can only be either or. Now it gets hard. What I do is try the options when it can only be one of two numbers. Sometimes you have to 'carry' up to four or five possible squares resolutions until you find that you have a square or column that cannot resolve if you choose that first option, so the other option must be the one. No cheating is allowed, it all must be done in your head. It is a great short term memory exercise. This weeks Sunday Times Hard No. 992 and Very Hard No. 993 I get as, reading top to bottom, the shaded squares to be 6,6,3. and 5,8,8.