well, the first and most important answer is practice. everybody will say that.
secondly, really, you should use the search function to find answers for things like this - there are many very similar threads. but im feeling charitable, and i am ill and off school so i have spare time...
cross - try and see the shortest solution possible - often you will be able to find several. try and use your ADF (adjustment of D face) wisely - solve edges relatively to each other, not just line up with centres and put in. solve the cross on its own over and over again - it will come with practice.
also, try and work out where f2l pieces will be after you have finished the cross - make note of (or even force) some 'stuck pairs', just so you are prepared to instantly start on a pair after your cross.
be able to solve blindfolded every cross you plan - this way you do not have to concentrate on the moves you are making and the transition between cross and f2l will be a lot easier.
f2l - constantly look for corners and edges. do not concentrate on the f2l pair you are doing - with enough practice and familiarity, you will be able to do this almost unconsciously. its a bit like... playing the piano - you are doing two things at once, but with enough practice it becomes very easy.
do NOT worry about your execution speed - as long as you are smooth then thats ok. practice doing OH and slow/smooth - turning solves to practice this.
LL - learn all the PLLs. seriously, it is not half as hard as it looks. for example, if you know the J permutation, you know 3 other PLLs too - mirror it to get J'. add RUR'U and do a J, then undo those set up moves, and you have an N-perm. mirror that, and you know the second N-perm.
there are lots of tricks like this.
also, try and use a little LL edge control - if you have the UF and UR edges unoriented as you do the last pair, instead of inserting it with RUR' or whatever, put the pair over the slot and do RFR'F', or the mirror (with edges UF + UL). this also prevents you from getting Dot cases, which are horrible when doing 2 look OLL.
also, vary between RUR' and RU2R' on the last insertion to change what OLL type you will get.
you can find plenty more stuff to learn if you search around. hope this helps.
and USE THE SEARCH BAR.
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